A little embarrassed that I did not know this, even though I have used PDFs for decades, and have always been frustrated with them because every time you re-open a PDF, you end up right at the first page of the document and have to try and remember where you were the last time. Really a pain when reading a long document or book that will take more than one sitting to get all the way thru.
I've always looked for a way to "bookmark" the current page, just like you would in a book. But Adobe had other uses for the word "bookmark" when they created Adobe Reader. A "bookmark" to them is to create one of many shortcuts in their "bookmarks" list. Even worse, you cannot create one in the free version of Acrobat Reader. So that sucks.
So how do you do this with the free version of Adobe Acrobat Reader?
It's embarrassingly simple, but this is all you have to do: Click on Edit, then Preferences, then Documents, then put a checkmark in for "Restore last view settings when reopening documents". Click OK.
Scroll down to some page, close Reader, re-open, and VOILA - you are right at the page you were at when you closed the document.
(Damn Adobe, that's some terrible User Experience there. That should be front-and-center. I can't think of anything more important in a book metaphor than how to mark your last page read. Normally, you use a bookmark with a physical book. but with a PDF, that's counterintutively not what a bookmark does. That's terrible and stupid. Do better! Be better!)