In addition to the known issues with the document browser on iOS 11, there are issues with third-party cloud providers where some files can't be downloaded, or are missing, or get corrupted.
You can temporarily copy the files into the local storage of PDF Viewer or iCloud while you are working on them, and then export them back into the preferred cloud storage when you finish editing, like so:
- Open the document you want to edit
- From the top toolbar, select the Export button, which looks like this:
3. Tap “Export” at the bottom of the window
4. Select either “On My iPad” or “iCloud Drive” as your location. Then tap “Add” on the top right
5. Once you are finished editing the document, you can export it back into your external cloud storage using the same steps