Besides ElementBuilder/Writer another cross-platfrom option to add new content to existing PDF pages is with
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pdftron.PDF.Stamper: www.pdftron.com/pdfnet/samplecode.html#Stamper. Stamper is simpler to use but less flexible as low-levelElementBuilder
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by creating a temp PDF using any PDF generation option at your disposal and placing the content on top of existing PDF page(s) as shown in ImpositionTest sample: http://www.pdftron.com/pdfnet/samplecode.html#Imposition