Of course, it depends on your PDF file. Many are designed for reading on a computer screen or printing out at A4. The best way to read if the text is too small is to turn them through 90 degrees and switch to continuous mode, then turn your device sideways too. Or if the document has wide margins, you can zoom so the text fits across the page and switch to continuous mode.
If you've got documents in Word or some other word processor format (such as RTF) the "How to make content for your document" file in the Docs folder will tell you the best page size to save PDFs at and how to do it. For ebooks that aren't already PDF, I use OpenOffice.org Writer to format to the right page size, then save as PDF.
And all the books at www.feedbooks.com
can be downloaded in a PDF format suitable for the iLiad (Feedbooks are formatted much better than manybooks, though).
There are also some alternative PDF readers available that allow things like full screen display and column following, but you'll probably want to get more familiar with your new gadget as it is before you take the plunge of installing the developers' package and adding new software. There's lots of people here who'll be able to help with that.