<PAGE>   1
                         [DOCUMENT SCIENCES LETTERHEAD]


                                                                   EXHIBIT 10.28

September 30, 1998

Mr. Thomas Anthony
2 Rollingview Lane
Fallbrook, CA  92028


RE:  SEPARATION AGREEMENT AND RELEASE OF CLAIMS


Tom,

This letter agreement confirms the mutual understanding and agreement between
you and Document Sciences Corporation (the "Company"), regarding your
resignation as an officer and employee of the Company as follows:

Resignation. You have resigned from your position as Vice President - Sales, and
any other title or position, and as an employee of the Company effective
September 30, 1998 (the "Resignation Date").

Company Payments and Benefits.

(a) in consideration of the mutual promises and agreements described herein and
    as severance pay and in lieu of any other monies payable, the Company will
    pay you compensation in the amount of one month's salary (less applicable
    withholding for taxes), payable in a lump-sum cash payment on the effective
    date of this agreement. From and after the Resignation Date, you will not be
    entitled to accrual of any employee benefits, including but not limited to,
    vacation benefits or bonuses, and you will not participate in any executive
    incentive compensation or bonus plan, or any other officer or employee
    compensation plan.

(b) the Parties agree that the exercise of any stock options shall continue to
    be subject to the terms and conditions of the Company's Stock Option Plan
    and the applicable Stock Option Agreement between you and the Company.

(c) for a period of 18 months after the Resignation Date, or shorter if you
    terminate such arrangement in writing with the Director of Human Resources,
    you will be entitled to continue to receive 100% Company-paid health, dental
    and vision coverage as provided to you [and your dependents] immediately
    prior to the Resignation Date (the "Company-Paid Coverage").


<PAGE>   2

Reimbursement of Expenses. The Company will reimburse you for all applicable
business expenses incurred by you through September 30, 1998, in the course of
performance of your duties for the Company, in accordance with the Company's
expense reimbursement policies. Such expenses must be submitted within 30 days
of the effective date of this agreement.

Confidential Information. You will continue to maintain the confidentiality of
all confidential and proprietary information of the Company and will continue to
comply with the terms of any proprietary information agreement, confidentiality
agreement or similar agreement between you and the Company.

Return of Property. Within five days of the Resignation Date, you will return to
the Company and physical or intellectual property of the Company or any property
leased or rented by the Company in your possession. Per the terms of your
agreement with Al Lynchosky and Chuck Harris, the Company has agreed to allow
you to retain your computer.

Acknowledgment and Release of Claims. Except with respect to the express
obligations of the Company as set forth in this Agreement, you agree that the
foregoing consideration represents settlement in full of all outstanding
obligations owed to you by the Company and that you will not seek any further
compensation from the Company for any other claim, damage, cost or attorney's
fees. The Company on one hand and you on the other hereby fully and forever
release each other and each's respective heirs, family members, board members,
executors, officers, directors, employees, shareholders, administrators,
affiliates, divisions, subsidiaries, predecessor and successor corporations, and
assigns, form, and agree not to sue concerning, any claim, duty, obligation or
cause of action relating to any matters of any kind, whether presently known or
unknown, suspected or unsuspected, that either you or the Company may possess
arising from any omissions, acts or facts that have occurred up to and including
the effective date of this Agreement including, without limitation:

(a) any and all claims relating to or arising from your employment relationship
    with the Company and the termination of that relationship;

(b) any and all claims relating to, or arising from, your right to purchase, or
    actual purchase of shares of stock of the Company, including, without
    limitation, any claims for fraud, misrepresentation, breach of fiduciary
    duty, breach of duty under applicable state corporate law, and securities
    fraud under any state or federal law;

(c) any and all claims for wrongful discharge of employment; breach of contract,
    both express and implied; breach of a covenant of good faith and fair
    dealing, both express and implied; negligent or intentional infliction of
    emotional distress; negligent or intentional misrepresentation; negligent or
    intentional interference with contract or prospective economic advantage;
    defamation; negligence; personal injury; assault; batter; invasion of
    privacy; false imprisonment; and conversion;


<PAGE>   3
(d) any and all claims for violation of any federal, state or municipal statute,
    including, but not limited to, Title VII of the Civil Rights Act of 1964,
    the Civil Rights Act of 1991, the Americans with Disabilities Act of 1990,
    the Family and Medical Leave Act, the Fair Labor Standards Act, the
    California Fair Employment and Housing Act, and Labor Code section 201, et
    seq.;

(e) any and all claims arising out of any other laws and regulations relating to
    employment or employment discrimination; and

(f) any and all claims for attorney's fees and costs.

You agree that the release set forth in this section shall be and will remain in
effect in all respects as a complete general release as to the matters released.
This release does not extend to any obligations incurred under this Agreement.

Acknowledgment of Waiver of Claims, including Claims Under ADEA. You acknowledge
that you are waiving and releasing any rights you may have against the Company,
including claims under the Age Discrimination in Employment Act ("ADEA"). [You
and the Company agree that this waiver and release does not apply to any rights
or claims that may arise under the ADEA after the effective date of this
agreement.] You acknowledge that the consideration given for this waiver and
release is in addition to anything of value to which you are already entitled.
You further acknowledge that you have been advised by this writing and
understand and agree that:

(a) you are advised to consult with an attorney prior to executing this letter
    agreement;

(b) you have carefully read and fully understand this letter;

(c) you are, through this letter agreement, releasing the Company from any and
    all claims you may have against it;

(d) you knowingly and voluntarily agree to all of the terms set forth in this
    letter agreement;

(e) you knowingly and voluntarily intend to be legally bound by the provisions
    of this letter agreement;

(f) you have at least twenty-one (21) days within which to consider this letter
    agreement;

(g) you have a full seven (7) days following the execution of this letter
    agreement by the parties to revoke this letter agreement; and

(h) this letter agreement shall not be effective or enforceable until the
    revocation period has expired.


<PAGE>   4

Civil Code Section 1542. You and the Company represent that the parties are not
aware of any claim by either of them other than the claims that are released by
this Agreement. You and the Company acknowledge that the parties have had the
opportunity to consult legal counsel, have read, are familiar with, and
understand the provisions of California Civil Code Section 1542, which provides
as follows:

               A GENERAL RELEASE DOES NOT EXTEND TO CLAIMS WHICH THE CREDITOR
               DOES NOT KNOW OR SUSPECT TO EXIST IN HIS FAVOR AT THE TIME OF
               EXECUTING THE RELEASE, WHICH IF KNOWN BY HIM MUST HAVE MATERIALLY
               AFFECTED HIS SETTLEMENT WITH THE DEBTOR.

You and the Company, being aware of said code section, voluntarily agree to
expressly waive any rights each may have thereunder, as well as under any other
statute or common law principles of similar effect, including both known and
unknown claims.

Publicity.  The Company and you agree to use your respective best efforts to
maintain in confidence the contents and terms of this Agreement.

Disparagement. The Company and you agree to refrain from any disparagement,
criticism, defamation, slander of the other, or tortious interference with the
contracts and relationships of the other.

No Admission of Liability. You understand and acknowledge that this Agreement
constitutes a compromise and settlement of disputed claims. No action taken by
you or the Company, either previously or in connection with this Agreement,
shall be deemed or construed to be (a) an admission of the truth or falsity of
any claims heretofore made or (b) acknowledgment or admission by either party of
any fault or liability whatsoever to the other party or to any third party.

Severability. In the event that any provision hereof becomes or is declared by a
court of competent jurisdiction to be illegal, unenforceable or void, this
agreement shall continue in full force and effect without said provision.

No Oral Modification. This Agreement may only be amended in writing signed by
you and a duly authorized officer of the Company.

Counterparts. This Agreement may be executed in counterparts, and each
counterpart shall have the same force and effect as an original and shall
constitute an effective, binding agreement on the part of each of the
undersigned.


<PAGE>   5

Voluntary Execution of Agreement. This Agreement is executed voluntarily and
without any duress or undue influence on the part or behalf of the parties
hereto, with the full intent of releasing all claims. The parties acknowledge
that:

(a) They have read this Agreement;

(b) They have been represented in the preparation, negotiation, and execution of
    this Agreement by legal counsel of their own choice or that they have
    voluntarily declined to seek such counsel;

(c) They understand the terms and consequences of this Agreement and of the
    releases it contains; and

(d) They are fully aware of the legal and binding effect of this Agreement.

Arbitration. The Parties agree that any and all disputes arising out of the
terms of this Agreement, their interpretation, and any of the matters herein
released, shall be subject to binding arbitration in San Diego County before the
American Arbitration Association under its California Employment Dispute
Resolution Rules, or by a judge to be mutually agreed upon. The Parties agree
that the prevailing party in any arbitration shall be entitled to injunctive
relief in any court of competent jurisdiction to enforce the arbitration award.
The Parties agree that the prevailing party in any arbitration shall be awarded
its reasonable attorney's fees and costs.

Other. This Agreement shall be governed by and construed in accordance with the
laws of the State of California applicable to agreements made and to be
performed entirely within this state.

This Agreement represents the entire understanding and agreement between the
Company and you concerning your separation from the Company as an officer and
employee, and supersedes and replaces any and all prior agreements and
understandings concerning your employment relationship with the Company and your
compensation by the Company.

You represent that you have carefully read and understand the scope and effect
of the provisions of this Agreement, and that you have not relied upon any
representations or statements made by the Company that are not specifically set
forth herein. You acknowledge that you have had the opportunity to consult with
legal counsel regarding the provisions of this Agreement and that you understand
the terms and effect of this Agreement.

This Agreement shall be effective and enforceable seven (7) days after it has
been signed by you and the Company.


<PAGE>   6

If the foregoing correctly sets forth our understanding and agreement, please
sign the duplicate enclosed copy of this letter Agreement in the space provided
below, and return a fully signed copy to me.

Very truly yours,

Document Sciences Corporation




By: /s/ CHARLES R. HARRIS
   -------------------------------
        Charles R. Harris
        President & CEO



ACCEPTED AND AGREED TO
AS OF SEPTEMBER _______, 1998




----------------------------------
Thomas J. Anthony

Should you have any additional questions, please do not hesitate to contact me
directly at (619) 643-5258.

Best Regards,

/s/ BILL DOWLER

Bill Dowler
Director, Human Resources