Note that while manipulating the ContentStreams collection (inserting, removing or moving elements) is allowed, the result of such manipulations may produce an invalid PDF if the original document was created or modified by a tool other than GcPdf. Generally speaking, it is safe to insert new content streams at the first position in the collection, or add them to the end of it, provided any content in the new stream(s) was created by rendering on PageContentStream.GetGraphics. Any other operations on this collection in documents that were not created by GcPdf might yield invalid PDFs.