Well, start here: https://www.damiensymonds.net/2010/05/best-practice-for-web-photos.html
I think you think this is more complicated than it really is.
Just resize using Image Size, then sharpen and save.
Well, none of that is any different with your R5 than it was with your previous cameras. It's ALWAYS been necessary to shrink files for email and Facebook.
How have you done it in the past?
You're absolutely correct, PDF is the right (and only) way to save.
However, if they're "way too big" there must be something else wrong.
What kind of documents are you creating, exactly?