create a fixedReadonly doc by importing readonly pdf
create a fixedFillable doc by importing fillable pdf
fixedReadonly.Merge(fixedFillable)

pdfformatprovider.export(fixedReadonly) throws the error below.
but vice versa works.
but i need the fixedReadonly pages and then the fixedFillable pages in the exported pdf.

any idea?

Value cannot be null. Parameter name: context
source:Telerik.Windows.Documents.Fixed
stack trace:
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Export.ContentElementWriters.ContentElementWriter`1.Write(PdfWriter writer, IPdfContentExportContext context, Object element)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Forms.VariableTextObjectPropertiesExporter`1.ExportVariableProperties(VariableTextProperties textProperties)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Annotations.WidgetPropertiesExporter.ExportWidgetProperties(Widget widget)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Annotations.WidgetObject.CopyPropertiesFrom(IPdfExportContext context, Widget annotation, RadFixedPage fixedPage)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Annotations.AnnotationFactory.CreatePdfAnnotation(IPdfExportContext context, Annotation annotation, RadFixedPage fixedPage)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.DocumentStructure.Page.CopyPropertiesFrom(IPdfExportContext context, RadFixedPage fixedPage)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.DocumentStructure.DocumentCatalog.CopyRadFixedPageProperties(RadFixedPage source, Page destination, IRadFixedDocumentExportContext context)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.DocumentStructure.DocumentCatalog.CopyPagePropertiesFrom(IRadFixedDocumentExportContext context)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.DocumentStructure.DocumentCatalog.CopyPropertiesFrom(IRadFixedDocumentExportContext context)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Export.PdfExporter.Export(IRadFixedDocumentExportContext context, Stream output)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider.ExportOverride(RadFixedDocument document, Stream output)
   at Telerik.Windows.Documents.Common.FormatProviders.FormatProviderBase`1.Export(T document, Stream output)
   at Telerik.Windows.Documents.Common.FormatProviders.BinaryFormatProviderBase`1.Export(T document)
   at PTMS.ReportLibrary.BaseAttachment.MergeDocument(RadFixedDocument radFixedDocument)

Hi Remzy,

I noticed this thread is the same as the one here: Value cannot be null parameter name: the context in Telerik Document Processing PdfProcessing - Telerik Forums. 

We can continue to discuss this there. If you re willing to share the documents that you are using I can convert this one to a support ticket which is a private thread. This way I will be able to determine what exactly is causing the exception. 

I am looking forward to your reply.

Regards,
Dimitar
Progress Telerik