So I have the same issue as described in
https://stackoverflow.com/questions/415 ... ertain-pdfFor my pdf that's exhibiting this behavior, I can tell that the pdf was scanned with an image on top of OCR text that isn't visible. When I add my text stamp using DrawString, it is there but also not visible. I can copy the contents I wrote. A subsequent reopening of the file and DrawString will now show visible text.
The workaround of always stamping twice won't work for me. The second stamp isn't in the same position as the first one so the text is "thicker".
Any ideas on a work around or how I can detect this situation so I can selectively stamp it twice?