This issue affecting FileApp is due to a limit in iOS that restrains the 'Open In' list to show 10 apps that can handle any given file type. This problem can happen for example when opening PDF attachments from Mail App or Safari downloads.
As PDF is commonly handled by numerous Apps it is very likely that more than 10 Apps are compatible with the file format you are trying to send to FileApp.
To solve that, delete other Apps you do not use for that specific file format and restart the device.
You may reinstall the other apps after having solved the issue.