I need to combine file pages from FileNet into a tiff format in C# WPF
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This code will get me the first page of a file from FileNet file and convert it to a tiff.  

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    string fileNm = fnDoc.GetCachedFile(1, "", 
                            IDMObjects.idmGetCachedFileOptions.idmDocGetOriginalFileName);
    string name = fnDoc.Name;            
    string onlyName = fnDoc.Name;
           
    m_destFile = pathStart + docSubPath + name + ".tif";

    File.Copy(fileNm, m_destFile, true);
    File.SetAttributes(m_destFile, FileAttributes.Normal);

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How do I instead, loop through the pages and then combine all the tiff's into one tiff file with multiple pages?