Replied 24 September 2020, 10:52 pm EST
I believe I have the issue resolved and all in working order.
To summarize for anyone else stumbling across a similar issues:
The original version of the WebForm application was utilizing C1Report and utilizing the C1Report method RenderToFile to export to a PDF.
My original issue in attempting to migrate from C1Report to C1FlexReport was occurring when attempt to load the report definition: with the error 'C1.Win.FlexReport.Serialization.C1ReportSerializer threw an exception'.
Further investigating\details of the error indicated I needed to include C1.Win.C1FlexChart.4.dll and C1.Win.C1DX.4.dll in the bin folder. Once included the report definition loaded without errors.
The updated version of the WebForm application now utilizes C1FlexReport and Renders \ Exports via the C1Document to a PDF.
Thanks Manish for the assistance and pointing me to the correct solution.