PDF Reader, for oldsters like me, means Acrobat because at the bzginning, there was an Acrobat Reader
Yes, it would be so nice tpo be able to do that.
In a software, I added more than Help:
I add window help button (specific to that window, in a Canvas with a “i” icon);
I also add “below the Mouse" a “one liner help” à la Xojo IDE when the Mouse is above a line of code (an icon in the Properties pane, etc.).
I also simply add that for my own; sometimes I totally forgot how my software works !