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;
}

When exporting documents containing TrueType Postscript fonts perceived as Standard fonts the fonts should not be embedded but they actually are.

Examples of such fonts are Arial MT, TimesNewRomanPSMT, etc.