The answers above are technically correct but the OP is asking a common question for those new to the iPad, i.e. Where's the common file system I'm accustomed to looking at on my computer? The answer to that question is that it does not exist. "Files" are not stored on disk and available for multiple applications to access. Instead, the data associated with an app is "private" and managed by that app. There are some qualifications to the following, but basically it means that every application has a separate copy of a file to which it has access.
This can be an inefficient and frustrating approach to file management and the best way around it is (as described above) to use an external data store (such as Dropbox or now iCloud) and a specific application like Good Reader to access the file. It's not a complete solution because as you can guess, you often end up with the same content stored in different files. (This is especially obvious if you try to create multiple photo albums; there is simply no good way to put a photo in one album and remove it from the "Camera Roll" where almost every image is deposited, whether a photo or not.) You can get around the problem to some extent by backing up files on a computer (either Mac or Windows) and manipulating them there and then sync with your iPad but it's a well-known PITA.
So why does Apple do it this way? Two answers. First, it is Apple's belief that managing a file system is not something those who use iOS should be engaged in doing. And for many iPhone users, they're probably right. It does introduce complexity in the way one thinks about the device. It makes less sense when you're talking about the iPad but since the iPhone and iPad share an OS, iPad users (without a jailbroken device) are forced to live with it.
There is another reason, however. And it's a somewhat more defensible one. By forcing individual apps to manage their own data, iOS places an app and all its data in what is sometimes called a "walled garden" where it is very difficult to imbed "weeds" (malware and viruses) that can "get out" into the rest of the OS and infect other apps and the OS, itself.
It does take getting used to and many of us who are used to a very different system model are constantly frustrated by it. But it's really only one step further down a road that has been around for a long time. Note, for example, in Windows that a particular application e.g. Word can only "see" (by default) files that end in .doc or .docx. The iPad approach is a bit more extreme. Not only can an app not "see" data objects that it cannot manipulate, it can only see the data objects that it has created or stored for itself.
You'll probably get used to dealing with the different model, especially if you use a cloud-based data store (e.g. Dropbox or iCloud) and a somewhat more powerful app that can at least "see" and manipulate (to a limited extent) data it did not create, i.e. Good Reader. (For the record "Good Reader" is a terrible name for a very powerful tool. It's the Swiss Army Knife of iPad apps. And if you don't yet have it. Get it. )
And after that needlessly long-winded answer to your question, here's the short version. If you get, say, Pages it will store word processing documents that you can then access with Pages. But unless you jailbreak your iPad (which you cannot yet do if you have iOS 5 or any OS version after 4.3.3) you won't be able to see or manipulate those documents with anything other than Pages. Same with Numbers. Same with Keynote (the Apple version of PowerPoint.) The ability to move between apps and import/export data created by one app with another is very limited. (Although if you use the Apple iWorks apps you'll have some of this capability.)