Pages and Numbers are suppose to import and save in Word and Excel but I think the more complex documents with macros and some complicated formatting won't work.
There are rumors that MS is working on iOS and Android versions of Office but they're probably going to try to boost sales of Surface and Windows 8 tablets with Office first before releasing on iOS or Android.
2. I don't think there's a direct way to do it. You can use things like DropBox or you can use app-specific documents. For instance, there are a lot of apps which can view PDFs and you would either load those through iTunes or email PDFs to an email account that you can open on the iPad and then it will give you options to load the PDF to some other apps.
Specifically, you can open PDFs in iBooks this way. There are some paid apps. which let you edit and/or mark up PDFs.
But other types of formats, like Office, would depend on certain apps. Like I said, Pages can open Word docs and Numbers can open some Excel files. Keynote probably can open some PPT files.
Photo apps like iPhoto and Photoshop Touch can open JPG and RAW photo files. There are rumors Adobe is going to release LightRoom for iPad.
I believe there is a version of Turbotax for iPad but I don't know how they send files between the PC and iPad versions.
I think the latest Windows version of Quicken will let you open your Quicken data in a free iPad app. Again, not sure how they send data over. Probably you have to log in through your Intuit account. Or they may use iCloud.
Other file types, it depend on whether there are apps. available to open those file types. Either you load the files through iTunes or the app. developer will provide another way.
But you're not going to be able to mount the iPad on your PC and drag and drop files, unless maybe if you jailbreak.
3. I haven't tried these but these promise some kind of drag and drop functionality, for copying files over from your computer to the iPad:
https://itunes.apple.com/us/app/files-app/id595075818?mt=8
https://itunes.apple.com/us/app/files-document-reader/id294150896?mt=8
But for complete file system access, you probably need to jailbreak. Now even if you do get full file system access, if you transfer certain file types, you of course need an app that can open and process those file types.
4. I doubt any printing app. would work in the background. Not sure, because I haven't really used it. All the printer vendors (HP, Epson, etc.) offer apps. which support some of their printers.
Just Google your model and see if there's a way to print from iPads.
There are some third-party printing apps. which claim support for wide range of printers, such as:
https://itunes.apple.com/us/app/print-print-to-all-printers/id299423224?mt=8
Obviously you have to have some kind of printer which is on the same network as your iPad will be, unless some apps. support USB printers which are connected to your router (if your router supports USB printing) or your computer.