PeachPDF
PeachPDF.PdfGenerateConfig
PdfGenerateConfig.TransparencyPolicy Property
What happens when a document that targets PDF/A-1 or PDF/X-1a/X-3 (all of which forbid transparency) uses something that needs it. Reject (the default) fails generation with an error naming the construct; Flatten renders the affected region into an opaque bitmap at RasterizationDpi instead (embedded as DeviceCMYK under PDF/X-1a). Has no effect at any other conformance level, which permit transparency.
public PeachPDF.TransparencyPolicy TransparencyPolicy { get; set; }