Office not opening pdfs on mobile devices

Describe the bug
We are experiencing an issue where shared PDF documents that have the “Download and Sync” permission disabled cannot be opened on mobile devices via Collabora.

On iOS, the client returns the error: “403 Forbidden – you don’t have permission to complete the operation."
On Android, the document does not open at all.

On Windows clients, the same documents open without any issues with collabora.
Word and Excel are working without any problems on android and apple, with this permission.

To Reproduce
Steps to reproduce the behavior:

1. Share a pdf file with a user and only allow to read it. “Download and Sync” permission set to disabled.
2. login with the user on android or apple
3. open the file
4. see the error “403 Forbidden – you don’t have permission to complete the operation.” on apple or on android nothing

Expected behavior
It should open the pdf as ready only in collabora.

Client details:

• OS: iOS 26.1, Android 16
• Browser [Brave]
• Device: [e.g. iPad 10th, Pixel 8]

Server details

Operating system:
Debian / Docker container

Web server:
Apache

Database:
MariaDB 10.6

PHP version:
8.3.30

**Nextcloud version:
31.0.13

Version of the richdocuments app
8.7.7

Version of Collabora Online
25.04.8.1

Configuration of the richdocuments app

{
    "apps": {
        "richdocuments": {
            "canonical_webroot": "",
            "disable_certificate_verification": "",
            "edit_groups": "",
            "enabled": "yes",
            "external_apps": "",
            "federation_use_trusted_domains": "yes",
            "installed_version": "8.7.7",
            "public_wopi_url": "https:\/\/colla.domain.de",
            "types": "filesystem,prevent_group_restriction",
            "use_groups": "",
            "wopi_allowlist": "internal ip adresses",
            "wopi_url": "https:\/\/colla.domain.de"
        }
    }
}

No source file or test is named. Start by reproducing the read-only PDF flow with “Download and Sync” disabled on iOS and Android, then trace the richdocuments/Collabora request that returns 403. Done means the PDF opens read-only on both mobile platforms without changing behavior for Word or Excel.