(Assuming H2O) It will depend upon the PDF. Documents with small fonts will be more difficult to read, even though the resolution is sufficient to render it clearly. The refresh rate makes panning a test of patience if you do want to zoom in. Complex documents will also be slow to render due to the limited processor.

That being said, there are patches to Kobo's firmware that improve the PDF reading experience. KOReader also offers a much better PDF reader. Then again, even the stock device will be able to handle a PDF file with larger fonts and a relatively simple structure just fine.