Calibre PDF at Synology with DOCKER

[Verdi-Fan]

Good evening everyone!
I have a strange problem. I have (betwenn many epub files) saved around 117 PDF files in Calibre 7.21
On the local Windows 64bit installation, all PDF files open immediately with the external program Acrobat Reader. No problem.
On the Synology Docker installation (we are taking always aboaut Calibre 7.21), the PDF cannot be opened with any (!) external program. If I switch Calibre PDF to internal programm, then about half of the PDF file opens with the PDF-viewer. The other half does not open at all - only later (next step) with Calibre-WEB (i.e. the library interface) it works fine. There we go again.

I would like to manage all PDF with Calibre 7.21 at Synology 224+ - but how can I do? Is there any possibility to start all PDF Files intern or extern?

Thx
VF