Q:

Email a existing pdf file with password protection

I have created a pdf using itextsharp.  The requirement is that while sending the PDF as attachment in an email and it should be password protected. 
I know that we can make a pdf password protected while creating it using itextsharp.  
Is there any way to make an existing pdf password protected? 

A:

Use PdfReader in combination with PdfStamper and the setEncryption() method. 
In Java, see encryptPdf() in this example: http://itextpdf.com/examples/iia.php?id=219
In C#, see EncryptPdf() in this example: http://kuujinbo.info/iTextInAction2Ed/index.aspx?ch=Chapter12&ex=EncryptionPdf

