Hallo Mario,
ok i got it working. The “Could not load file or assembly ‘System.IO.Packaging, Version=4.0.5.0” occures then you are using TargetFramework netcoreapp3.1 but referencing the GemBox.Pdf for net35. Referencing the GemBox.Pdf of the /netcoreapp3.1 folder solves it.
thanks and kind regards,
Sven