I'm experiencing the same problem (obviously, since the line is commented out this would impact anyone using the latest version). I tried building locally and un-commenting out the line as-is and it worked. I'd hate to have to use a forked version of PdfSharp in my project just to uncomment one line; it'd be swell if whenever the next release happens this is changed. The change I made is what PeterEm suggested. I un-commented out line 444 of PDFsharp\src\PdfSharp\Pdf\PdfPages.cs so it now looks like this:
Code:
#if true
// Do not deep copy annotations.
CloneElement(page, importPage, PdfPage.Keys.Annots, false);
#else
// Deep copy annotations.
CloneElement(page, importPage, PdfPage.Keys.Annots, true);
#endif
It's also not obvious to me whether passing "false" to that final argument is right or not; since the commented-out line had it as false that's what I did, and it worked in my very limited testing. I also checked version 1.32, and it has that argument as false as well (and the line is not commented-out).
I can submit a pull request on Github if that would help.