German umlauts (the characters 'Ä', 'Ö', 'Ü', 'ä', 'ö' and 'ü') will not be printed to a generated pdf file, but the other German "special" character "ß" works.

I use the trial version via the Telerik NuGet feed. A test program and my locally generated file are attached.

There is a wrong reference in the WPF .NET Core version of Telerik.Windows.Document.Fixed.dll that causes a FileLoadException with the message "Could not load file or assembly 'System.Drawing.Common, Version=5.0.0.0'"

Workaround: Add NuGet reference to System.Drawing.Common, version 5.0.0.0. Note: mark "Include prerelease" so you can see it in the NuGet Package Manager

A Link annotation must have an associated action, destination, or named destination specifying what should happen when the annotation is activated. When merging documents with invalid Link annotations, an ArgumentException is thrown with the message 'Value cannot be null. Parameter name: namedDestination'

Workaround: Remove the invalid annotations:

foreach (var annotation in document.Annotations.ToList())
{
    var link = annotation as Link;
    if (link != null && link.Action == null && link.Destination == null && link.NamedDestination == null)
    {
        (annotation.Parent as RadFixedPage).Annotations.Remove(annotation);
    }
}