Both my merge splitting add-ins Individual merge letters. and Merge and Split will merge to PDF and allow you to set a common password (I am working on a version of the former that will allow you to set individual passwords from the merge data source, but it will not be ready for a while).

The process requires the addition of (free) third party software PDFCreator 2.5.2 which is also linked from the web page. Anecdotal evidence suggests that it will also work with the more recent and more bloated version of the PDFCreator software, but I haven't tested it and recommend instead the version 2.5.2.

__________________
Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com