If you don't set a "user password" [needed for opening the pdf file] your security is very LOW. Software can decrypt it in milliseconds. After doing it your reader will have
full controll of your document.
What you want to do is very easy with Adobe Acrobat - that is why it's so expensive.
It's called "batching" or so [in my German version "Stapelverarbeitung"].
You can write scripts [adding pages, changing document information etc.,] & let it run on complete folder of files.
I don't know if other software like from foxit offers such features.