To start looking for a good pdf reader for the iPad I just started searching for PDF apps. You can narrow the search down to just iPad apps and you can sort by popularity or relevance. I chose to look for the best ratings. There were two that I found that both had high ratings. So far here the best free iPad pdf readers:
The free version.
Update: iBooks
Originally I did not include iBooks because it is very slow on the iPhone 3GS. I decided to open a pdf in iBook from the internet and it was surprisingly fast on the iPad 2. This may be due to the faster processor but it works jut as well as the other pdf readers I have mentioned. The one drawback to iBooks is the difficulty in transferring pdf’s from the computer to the iPad. I think the other pdf apps are easier.