RadPdfProcessing does not provide an option to convert flow document (e.g. DOCX) to fixed format, although we are working on implementing such functionality. At this point such conversion can only be achieved using the assemblies shipped with RadRichTextBox and the code you found on the forums.
As to the error you are experiencing, the PdfFormatProvider class that can export a RadDocument instance to PDF is located in the Telerik.Windows.Documents.FormatProviders.Pdf
assembly. Please refer to this help article
for additional information on the assemblies you need to add to your project in order to use the control.
It seems the provider instance you are creating is from the Telerik.Windows.Documents.Fixed.dll and relates to the new RadPdfProcessing library. Like I said, the library does not allow for conversion of flow documents for now, but you can follow this item in the public portal
to be notified when the feature is available.
Do not hesitate to contact us if you have any additional questions.
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