Hello,
Thanks for your inquiry.
Did you install our Spire.PDF from NuGet in your .NET Core app? Sorry that starting from Spire.PDF v6.9.16, we have removed the .NETCore dlls from the NuGet package. Thus, for .NET Core Apps, it actually uses the .NETStandard dlls by default, and our .NETStandard dlls do not support defining the DPIs.
To solve this issue, you could install the package (
Spire.PDF.NETCore 7.1.0) via NuGet to use our .NETCore dlls.
Sincerely,
Brian
E-iceblue support team