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);
    }
}