Unlike the .NET Framework one, RadPdfProcessing’s .NET Standard version does not offer a default mechanism to read fonts. The FixedExtensibilityManager class is exposed to help implement this functionality.
PdfProcessing needs to have access to the font data so that it can read it and add it to the PDF file. That is why, to allow the library to create and use fonts, you will need to provide an implementation of the FontsProviderBase abstract class and set this implementation to the FontsProvider property of the FixedExtensibilityManager.
You can find a detailed FixedExtensibilityManager and FontsProvider descripiton and implementaion in the How to implement a FontsProvider article.
If the FontsProvider property is not set, a default font will be used when exporting the document.