When you try to load а stream from an absolute Uri with WebClient, the resulting stream is ConnectStream which does not support the Seek operation. This operation is vital for PdfFormatProvider, so it throws an exception. We will make sure to update our documentation about this limitation.
In these cases you can download (read) the content of the stream, create a new MemoryStream with it and pass it as a parameter to the PdfDocumentSource constructor. This, however, is a slow approach and is not recommended. You should use only documents that you can open a direct stream that supports Read and Seek operations.
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