robbieduncan is right, there's no way to get a list of all .pdf, .doc, or "dot anything" files on your phone. Have you seen the filesystem? I have, because I've used iFile extensively, and I can tell you it's very large. Not as large as on the Mac, but still very large. First of all, some directories your application can't even read. If you try to access the contents of it, say through NSFileManager, you will get an error because your app doesn't have permission to view those files. Secondly, to get a list of all "dot something" files on the phone, you would have to search through every directory on the phone, which would take a very long time and is not feasible.
The reason you can access all the photos stored in the photo gallery is because they are all stored in the same folder, which apple specifically allows you to access.
@raniwala25 O.K, there may be a "FileApp" which works on unjailbroken devices but what does it actually do? Does is just pull up all .pdf and .doc files? Or does it just let you browse directories? I'm telling you, there's no way to do this.