A strange symbol is added as a name when embedding a file with an empty string as its name argument.

For example exporting the text "\uD83D\uDE0A" with "Segoe UI Symbol" font family should export a single smiling face. Instead the characters are skipped during the export as PdfProcessing is trying to export them as separate char values ("\uD83D" and "\uDE0A") and the font does not contain glyphs corresponding to these char codes.

When importing specific documents and signing a predefined SignatureField leads to not successfully validated signatures in Adobe, but successfully validated in other pdf viewers (eg. Telerik WPF PdfViewer, Foxit PDF Reader).

Solution:
When signing an existing document (after the import) we must be sure the AcroForm's ViewersShouldRecalculateWidgetAppearances property is set to false, otherwise, the exported and signed PDF document could not be shown as a signed. 

if (document.AcroForm.ViewersShouldRecalculateWidgetAppearances)
{
	document.AcroForm.ViewersShouldRecalculateWidgetAppearances = false;
}

Observed: the following dialog appears:

They are exported with the /EmbeddedFiles switch. This would allow adding external files to the PDF document.