All ebook readers handle PDFs that way. The purpose of a PDF file is to preserve text. Adjusting fonts messes up that preservation. You need to stay away from ebooks in PDF format. I feel your pain. I have several library books that are only available in PDF. I have to just deal with the default small (S) text size, but the 900 has a margin crop feature. This gets rid of all that white space in the margins, hence making the text more readable. There's no option for this on the 600.
