If you are using Files format with the A5 Fax app, and some app users cannot search for PDF Files not owned by them when sending a fax, then you should check the File sharing permissions are not set to private. You can also check that each Salesforce user has Query All Files permission enabled in their profile to access the file permissions properly (see more details below) as well as the Content permissions.
If you are able to access Files already that are not owned by you though your faxes are stuck in “Sending” status, it’s possible that the user who authorized the app under Fax Configuration tab doesn’t have required file permissions. In other words, the user who authorized the app must have access to the files being faxed (the ones which are used as Sent Fax record’s Attachment ID fields). If that user is a creator and/or owner of the files, no additional configuration should be required. Otherwise, that user will need a Query All Files permission enabled in his profile for other app users to be able to access the file permissions properly.
Best way to configure this is by creating a permission set which adds Query All Files permission, and assigns it to the user which granted access for the app under Fax Configuration tab. You can also enable the permission for the whole profile, but in some cases, because of security and privacy reasons, it may be better to assign it to specific users using a permission set. If you have strict files permission setup in your org, you may need to create this permission set for each individual user that is accessing the files for faxes.
You can find this permission in the permission set under App Permissions —> Content —> Query All Files, and assign it by clicking on Manage Assignments and selecting which users to assign to. And ensure the Query All Files permission box is actually checked/enabled under user profile (see bottom visual screenshot). You may also need to enable the Content permissions by checking box “Manage Content Permissions.” After these action items, all users should be able to access files and send faxes successfully.
You can learn more about permission sets by following this link: https://help.salesforce.com/articleView?id=perm_sets_overview.htm&type=5