What is a good PDF app should i use?
Try iBooks; I also use GoodReader - but there are certainly other choices -
i use ibooks so far to read all my pdf magazines and not dissapointing although need a bit patience when it does updating after i add or delete pdfs from it i dont mind because it only happens during the 1st time after we did that after that loading normally but i dont know if other apps also needs to wait a bit after adding or and delete several pdfs during 1st time
If you use a lot of pdf's and also want to annotate them then Goodreader is the absolute best. This is one of my main apps. I use it daily and it's one of the big reasons I keep coming back to iPad. It's not expensive. Totally worth the price. Works with Dropbox which is really handy. I have a lot of patterns that I don't want to lose and I can upload and download inside Goodreader straight to and from Dropbox.
When it comes to large PDFs filled with diagrams and pictures I found out that iBooks renders pages faster than Goodreader. As a matter of fact, iBooks is faster than any app I've tried so far including specialized apps like PDF Expert or iAnnotate which is kind of strange.