Ebooks come in a variety of formats. Two of the most popular are PDF and ePUB. Sometimes you have the option to choose which format you want. Older PDFs are not reflowable so as to properly accomodate smaller screen sizes like those on a mobile device. So, if you are having trouble with the document fitting properly within your screen, try using an epub file if you have that option. Newer PDFs and various apps are designed to fix this problem.
Here's a quick blog entry by Deniz Oguz about pdf vs. epub for android.