Some Pdfs shared by a link are not readble

I would like to share some strange behaviour from OC 8.1.3.
OC is running on a shared host, simple Webinstall with HSTS.
The encrypted filesystem is choosen

We can upload files without Problems.
However when we share PDF’s with a link certain files cannot be read.
These were re-shared without effect, the user that made the link is an administrator.

Questions,
does anyone had somethong similar?
If I should update to what version and can it be done by overwriting the php-files with an upload?

I would be greatfull for any hint in the right direction.

Regards,
Jan

From the template shown when creating a new thread:

make sure that you’re running the latest available version for your oC release: Changelog - ownCloud

Hi

the updates have been made we are now running 8.2.7.

PDFs and also ODT load in an iFrame and my guess is that Firefox, but also some other OS-Browser combinations forbid to load the images (IE 11 on Win8.1 shows them).

Does anyone have an idea what to “open up” in order to see the images?

I am searching all kind of fora to see what I can find.

Regards,
Jan