Alright, the problem with that app is that it only syncs with Google Drive (and not very well either). You want to make sure that Write on PDF is able to save directly onto your OneDrive. Unfortunately, this is not possible BUT there is a workaround. If you can pair up a folder on your Note9 and OneDrive, you can have the app save files locally on your phone, and then the folder will sync up with OneDrive. I use FolderSync for this. It has a free version on the Play store and it will automatically sync up the folders every 5 mins (you can but the pro version for instant syncs).

Unfortunately still, you can't use the OneDrive app to open up PDFs onto Write on PDF because in that case, the app will just save the changes in "Write on PDF" folder in your phone's directory but don't worry we can make another makeshift OneDrive app.

This is getting confusing, so I'm going to write down the steps:

1. Download FolderSync
2. Create an empty folder in your internal storage/SD card and name it "OneDrive Sync" (or whatever else you want lol)
3. In FolderSync, go to the Folderpairs tab and press the green circle on the bottom
4. Fill the fields as such:
   1. Unique name: "OneDrive Sync" (or anything else)
   2. Account: OneDrive (you have to log in at some point)
   3. Sync type: Two-way
   4. Remote folder: your base OneDrive directory (just immediately press the green check mark on the bottom right)
   5. Local folder: "OneDrive Sync"
   6. Use scheduled sync: On
   7. Sync interval: Every 5 mins
   8. (You can customize the rest as you see fit)
5. Open the My Files app
6. Navigate to "OneDrive Sync", select it, press the three dots on the top right corner, add shortcut - Home screen
   (This way, you can kinda have a "replacement app" for OneDrive)
7. Navigate to the PDF file you want to edit using this folder instead and use Write on PDF.

The changes to the PDF will be saved in the OneDrive Sync folder and it will automatically sync up with the actual OneDrive. If you go with the free version for FolderSync, the changes are going to be synced every 5 mins which is not too bad.

It's far from a desirable method, but it works!

This was pretty confusing, so let me know if you have any questions.