@linnygal: it depends on how the "conversion" is made. Basically if the texts are scanned as images you will need to "zoom in", as the reader can't understand there is text there. as austie333 pointed out, a pdf made by word or other pdf authoring software should have text saved as is, so the font/character can be enlarged natively just like any other ebook format.
I just received mine and I can confirm the latter, as Sony user guide comes in pdf. I'll see if I can find some scanned pdf and view that on my 650. I'll get back here to post my results.