Nitro PDF (the free version) fixes Acrobat Could Not Save error 18

As explained here (https://forums.adobe.com/thread/1028049), NitroPDF can fix some broken PDF files. In my case, Acrobat Pro XI refused to save a PDF file, saying it had error 18. After reading that previous tip (which saved the file normally but didn't fix my problem), I opened the file in Nitro PDF, then clicked File -> Save as -> Flattened PDF instead, and voilà, the PDF worked normally again! Yay!