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);
    }
}

I have read that there limitations to PDF files being imported, however, the file being imported  was generated with FixedContentEditor, so, should be able to import?

By specification, the last line of the file contains only the end-of-file marker, %%EOF. If the file contains many trailing bytes at its end, a NotSupportedException: 'StartXRef keyword cannot be found.', is thrown.

Workaround: Trim any content after the end-of-file marker (see FixInvalidEndOfFile). 

When merging documents` pages using the PdfStreamWriter the Form Fields are not copied:

using (PdfStreamWriter fileWriter = new PdfStreamWriter(File.OpenWrite(document2Name)))
{
	// Iterate through the files you would like to merge 
	for (int i = 0; i < 2; i++)
	{
		// Open each of the files 
		using (PdfFileSource fileToMerge = new PdfFileSource(File.OpenRead(document1Name)))
		{
			// Iterate through the pages of the current document 
			foreach (PdfPageSource pageToMerge in fileToMerge.Pages)
			{
				// Append the current page to the fileWriter, which holds the stream of the result file 
				fileWriter.WritePage(pageToMerge);
			}
		}
	}
}

A possible workaround is to use the RadFixedDocument`s Merge() method: 

document1.Merge(document2);

When merging documents containing the same embedded font every time a document is merged the same font is repeatedly embedded in the merged document. This leads to very large files.

A possible workaround before this feature is developed could be to use the PdfProcessing model to import and merge the documents. After the documents are merged to iterate the document`s content in order to cache the fonts using their name and if there are duplicated names to set the already cached one (check the attached project). 

As a side note, the suggested workaround doesn't cover all possible cases (e.g. if there are fonts with a common name but different font sets (modified fonts or font subsets) this may lead to missing/different characters in the produced document). So in order to use this workaround, you will need to ensure the font names are unique for every different font.

Support for exporting WMF (Windows Metafile) and EMF (Enhanced Metafile) images should be implemented for the Net Standard version.

To allow the library to export such images you can use the extensibility point for providing a custom implementation of the JpegImageConverterBase. More information and examples can be found in the PdfProcessing`s Cross-Platform Support help and Create Custom JpegImageConverter in .Net Standard knowledge base articles.