byte[] data = File.ReadAllBytes("verdana.ttf");
FontFamily verdanaRegularFontFamily = new FontFamily("Verdana");
FontsRepository.RegisterFont(verdanaRegularFontFamily, FontStyles.Normal, FontWeights.Normal, data);
FontsRepository.TryCreateFont(verdanaRegularFontFamily, out FontBase verdanaRegularFont);

foreach (RadFixedPage page in mergedDocument.Pages)
{
	foreach (TextFragment textFragment in page.Content.Where(c => c is TextFragment).Cast<TextFragment>())
	{
		textFragment.Font = verdanaRegularFont;
	}
}

Here is a sample structure of the signature: 

/Type /Sig
/SubFilter /adbe.x509.rsa_sha1
/Filter /Adobe.PPKMS

Corrupted document when exporting "Courier New" subset.

Workaround - fully embed the font:

var pdfFormatProvider = new PdfFormatProvider();
pdfFormatProvider.ExportSettings.FontEmbeddingType = FontEmbeddingType.Full;

NOTE: There are reports for other fonts' subset, e.g. "ZapfDingbats" font.

NetStandard: Missing content due to empty glyph outlines in embedded TrueType font subsets.

Workaround 1

Fully embed the fonts in the result PDF document:

PdfFormatProvider pdfFormatProvider = new PdfFormatProvider();
pdfFormatProvider.ExportSettings.FontEmbeddingType = FontEmbeddingType.Full;

Workaround 2

Switch to a .NET Framework project environment and use the .NET Framework PdfProcessing packages.

PdfStreamWriter stream-copy page splitting performance is much slower than expected in some cases. When working with larger PDF files, the processing time can increase a lot and cause noticeable delays.