Adobe reader reads .pdf's, doesn't it? Pdf is designed primarily to reproduce the exact appearance of a printed document, and is an entirely different animal from true ebook formats like .epub, which allow much more extensive formatting changes to suit individual tastes and screen sizes (if you can change your font size, type, margins and line spacing, who needs zoom?). If you want to find out more about how ebooks work, try the wiki at Mobileread. And again, to test reader apps, you should first make sure you have a correctly made file of a file type suitable for that app.
On a small screen it is hardly worth bothering with pdfs made from raw page scans IMO - if you're seeing finger marks, they are nothing but an image of a document and may not even have been converted to machine-readable text ("searchable pdf" which is a small step up), so can't be reformatted in any way except "zoom", which is likely to leave half the text offscreen once you blow it up big enough to read! Searchable pdfs are a bit better, but still are frustrating to read compared to other formats (of course you can at least convert those, but the ones from Google are full of errors...) (edited to add: rereading your post, it sounds like you have one of those Google books where they show an onscreen image of the raw scan, but let you download a searchable pdf version.)
On most true reader apps you can access the settings via the menu button on your phone.
