I create lots of PDFs of Excel solutions for my students. I secure these so that they are only hi-res printable and viewable by visually impaired but cannot be modified by the students. I use the same password always and that is adequate for me. I wish this could be made into a default password so that attaching and detaching the password is easier. In fact, I would like to be able to do it to say 20 PDFs at a time, using the multiple file feature presently available for adding header/footer, water mark and background image. Does this capability already exist in the Acrobat?
If you have Acrobat XI or DC Pro you can use Actions for what you want to do.