I think you will be mostly out of luck and/or eventually totally frustrated because ...
* You want to tag files
* You are using a mixture of WindowsOS and iOS
* You want to use iTunes only
This combination sets somewhat conflicting constraints. A built-in file tagging system does not exist on iOS, and the file tagging systems in the Windows and Mac OS worlds are different (meaning, when built-in file tagging would come to iOS, it will likely be incompatible with Windows). Based on trends with how Apple is pushing iCloud, file transfers via iTunes will soon be a thing of the past. You might find something that works now, but it will be unsupported in a very short time.
I might also add that, while the app TagSpaces looks promising, its approach seems to be building its own grave in some ways. Tags are added to the file name. This is certainly an open source, platform-agnostic method, yet I can only imagine that it is prone fail once file name limits are exceeded. From what I see, it also means that a file that is named "My Favorite Book".pdf on your Windows system will then become "My Favorite Book [biography Abe Lincoln civil war ....]".pdf once you tag it on your iPad and transfer it back. Either you will end with an over-written file or a duplicate file. How reasonable is this for you?
Finally, 32GB of storage on an iPad will get filled rather quickly with just the basic apps to run the iPad. How much file storage do you really need for all of your documents?
In the end, I suggest a different approach.
* Manage the file tagging entirely on the Windows side with an app that is comfortable for you (no need to buy a Mac for this).
* Store only a sub-set of the files on your iPad -- those that you are mark as "active" for reading
* Use the free limits of cloud storage to transfer files "active" and "done" files back and forth between the iPad and your Windows computer
This modified approach, where you divide the demands appropriately, will open the doors to a wider range of apps.
--
JJW
