Merging, which puts files together horizontally. Concatenation or Appending, which puts together files vertically.
Concatenation is generally a no-brainer but merging can be more complicated.
But remember, data wants to be bad. Say in your death certificate file you had some cases where a spouse's SSN was used. You might wind up with a duplicate. Sometimes these will be inconsistent. For example an ID might be '00005' on one file and '5' or ' 5' on another file.