How can I uninstall office word (ipad) while keeping my documents (in onedrive)?

One thing might be to open the document in one drive then 'open in another app', open in notes and save it in the notes app on your iPad. Which should break the link and then if you delete it from the iPad or delete word, the copy of the document should remain in notes.

To do what you want you want one drive to have a local copy of your file. I'm not sure if there is a way using the iPad to do that, but maybe if you access your one drive via your PC and see if there is a way to make a folder 'ipad only', which means it does not sync, so should be disconnected from Word if you delete it.

Deleting a document from Word on your iPad will of course delete it from OneDrive, because the documents are stored in the cloud based server. However, deleting the Word app from your iPad should not in any way affect the files that are stored on OneDrive. That is because by deleting the App, you are not in fact deleting the documents.

If you want to be absolutely certain, then make a copy of the files from OneDrive on a computer, and save them to a folder on the computer or a USB drive, or anything to make sure you don't lose them. Keep the originals in One Drive, and then delete the Word app form your iPad. Everything should be fine, but just in case it isn't, you'll still have a copy.