I have tried a lot of PDF Readers apps for my iPhone. But the best one so far has been PDF Reader Lite by Kdan Mobile Software LTD. There are many PDF Readers like iPDF, PDF Extreme, Reader Lite, and many more in the iTunes store but it seems most of them all have tried to renovate PDF Reader Lite and failed. It's a powerful file viewer too, which supports different formats like .doc, .txt, .ppt, .jpeg. It is very easy to use and powerful tool for your device but the Lite version only allows to store files no more than 10 files. If you like the Lite version you can consider to download the full version via iTunes Store.