Typefaces and Readability in PDF Documents
Type on a computer screen looks different than printed type. In order to provide the best viewing experience for readers of your PDF documents, you should constrain the use of typefaces to only those that look good on screen.
You can tell pretty easily what looks good on screen just by typing a few sentences and selecting different fonts right on your computer.
Things to avoid when choosing fonts for online viewing:
Big serifs. Fonts which have lovely large serifs look great when printed but terrible on a monitor; sometimes the serifs begin to resemble additional characters.
Fonts with drastically varying stroke weights. Some fonts have vertical strokes which are nice and thick, while their horizontal strokes are thin as can be, making them difficult to read on screen.
Massively thick fonts. If the "holes" in fonts are difficult to see on screen, they can reduce readability.
Also, try to avoid excessive tracking or kerning, and don't horizontally scale type less than 70%.