This project is a fork of https://github.com/YetaWF/PDFsharp-.netcoreapp2.0, which is itself a fork of the original PdfSharp. It is the only fork of PdfSharp I could find that supports .NET Standard and does everything I needed without errors.
PdfSharp project web site:
http://pdfsharp.net/
> Install-Package PdfSharp.netstandard -Version 1.3.2