support opening documents from folders other than ~Dokuments

I consider this as a "Won't fix" issue.
Document Viewer is not really intended to open something different from documents, and there may be some "limitation" (actually they are security features) in the platform itself if I go for implementing it.

Let me explain it better.
The current application model provides isolation between applications. That means that while importing a document (or any other file) from another application, the file is copied in another path, in order to avoid that some malware can edit files and/or access to sensible user's data.

Since you need just to view some file (and not to edit them), you actually don't need to view the original files.
There's anyway an issue here: a copy is created in HOME/Documents, and you need to manually delete it.
We can not workaround it, unless we implement a full-featured file manager in the app (and we'd need to support PAM autentication and other security stuff - breaking a lot the UX), but this won't fit with our plans for the project.
Any file imported through Content Hub is copied in a "trusted" folder, then imported in the app. No exceptions.

However there's an app that can access to the whole file system: the file manager.
Moreover, usually file managers also provide a basic text editor for such scenarios (e.g. ES File Manager on Android platform).

Being the docviewer just a document viewer, and the file manager a tool for advanced usage/user, this is a useful feature that filemanager-app could provide.

I'll mark filemanager-app as affected by this bug, and update the status as "Incomplete" for docviewer-app, since it needs more discussions with filemanager devs.

@File Manager devs:
I know you guys may not have time for implement this, but it may be worth to discuss about this issue.
I'm open to any discussion and/or collaboration for fixing this.

So the security model works in a way that an app can only access a limited set of files/user data?
Gallery can only access HOME/Picture, and DocViewer can only access HOME/Documents?
Did I understand that correctly?

Truth be told, I was just curious about how/where apps store their config, something normal user wont do normally. And I havent yet delved into deeper wiki readings about ubuntu touch's security policies.

Yes, it is. By default, an app can only access its own folder (for data it's usually $HOME/.local/share/<app_id>).
An app can set an additional set of location where it wants to have read and/or write access: for gallery-app it's user's Pictures folder, for docviewer-app it's user's Documents folder.
Third party apps that requires access to more locations on the file system will require a manual review while uploading in the store, core applications need to discuss about their needs with the security team.

We (docviewer-app team) could decide to read the whole file system, as filemanager-app or terminal-app do. But that wouldn't fit with the goals of the app.
It's a design issue, more than a engineering one, and we would need to do a lot of additional work because the platform *properly* force to respect some security rules.

You can find further information here: https://wiki.ubuntu.com/SecurityTeam/Specifications/ApplicationConfinement

To be fair I think you spotted a lack of features that would be nice to take care of.
IMHO the project that could satisfy this need is the filemanager-app, which operates in the typical scenario where this need gets 'disclosed' (i.e. advanced user that want to access to some system data - the file manager is the only user friendly solution that can do that).