When a style doesn't have explicitly defined font, the font from the default character properties (\defchp) is used when present. Instead, the font used for the style should be the default font for the document, defined with \deffN tag.
The same issue occur when the doesn't contain \defchp at all. In this case, the font for the style is not imported, but instead the defined with  \deffN tag should be used.

The construction is not common, and MS Word for example doesn't produce such documents.

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.

When a document containing a SignatureField is exported with the IsEncrypted property set to true, a not set UserPassword is required to open it, which makes it impossible to be opened. An InvalidOperationException is thrown with the message "Password is not correct." when importing with PdfProcessing; Adobe Acrobat also doesn't recognize the password.

Workaround: Exporting with AES256 encryption does not have this problem:

provider.ExportSettings = new PdfExportSettings
{
	IsEncrypted = true,
	EncryptionType = EncryptionType.AES256
};

• Coons patch meshes (type 6) construct a shading from one or more color
  patches, each bounded by four cubic Bézier curves.

Here is a sample structure of the signature: 

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