Two options come to mind.
- Why not just use Windows's built in features and Adobe Acrobat?
In Windows Vista, 7, and 8 tagging documents is really easy, and with group policy, so is providing access to folders (where the documents saved based on category.) Adobe Acrobat can easily search multiple PDF documents at once, and creating backups is as easy as copying the files to a USB drive (with or without software to do it automatically). And it's all 100% local.
Use Google Docs.
With Google docs, uploading and categorizing files is super easy and straightforward, and with Google Drive (all Google Docs are stored in the Google Drive) keeping an extra copy is also incredibly easy to setup. Google is arguably has the best searching algorithms, and will be able to index your files for you.
I hope this helps.