I upgraded to Adobe Pro DC about a week ago from the 2011 version. I have to combined PDFs, watermark, flatten, and secure them. I can combine the PDFs and save it just fine. However, whenever I try to do any work to it, it crashes. Specifically when flattening, it crashes but it will also crash when I try to add watermarks sometimes. I've chatted with Adobe twice and they've had remote access to my laptop. They say that there are too many pictures/fonts on the PDF, however, I've done this same process with my older version for years. I can watermark and flatten all other PDFs as long as I'm not combining them with another PDF. I've tried combining them via "save as" and in tools "organize pages and insert." Both ways still crash once I start working on the combined file. I'm working with Microsoft 2011 and my operating system is MacOS Sierra (the most updated version). I would appreciate any suggestions on how to fix this problem. Thank you.