What is the official general rule for this? PDF’s or PDFs?
What do you mean by “official”?
This is a question of style. You consult the style guide used for your publication. If you are not writing for a publication, you can decide your own style.
Personally, I prefer “PDFs.”
Chicago says only with a period or both capital and lower-case letters. MLA says no. APA says no.
I’m going to take a third option and say: PDF files.
I guess you missed the big fight about this raging in another thread.
In general, omit punctuation if it doesn’t serve a purpose. The apostrophe in “PDF’s” doesn’t serve a purpose (eg making pronunciation clearer, as in “A’s and E’s” versus “As and Es”), so I’d always leave it out.
Edit: unless, of course, the context is “the PDF’s properties” or “the PDF’s on its way”, or something.
In general, the apostrophe for plurals is being phased out. It only remains when the lack of it would cause confusion (e.g., “A’s” vs “As”).
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