I buy my ebooks in PDF because I can boost the font. Period.
I once had a friend comment that it was very hard for her to read the tiny font of one of my books. The 8 pt. font made her eyes water, as well as gave her a headache.
"Why don't you boost the font so it's easier to read?" I suggested.
"You can do that?"
Yes, you can if you're using Adobe (or any 0ther PDF reader, I would imagine.) Another neat trick I learned -- when it's time to scroll down, DON'T use the mouse. Put your cursor on the empty space ABOVE the bottom arrow, in that little "gutter" your scroll bar runs down. Then click once to advance the page down. The last line you finished will automatically become the first line at the top of the next page. :D
Ain't technology wonderful?