Save Adobe PDF compression resolutions are mixed up
When you go to Save As and select PDF, and go into the Compression tab there are three sections of settings for image qualities and resolutions, Color, Grayscale and Monochrome. The bug is that the Monochrome Bitmap settings are ignored and grayscale settings are used instead.
If you look at my PDF export settings screen shot you will see that Monochrome was supposed to export at 600dpi and Grayscale at 200. The Monochrome image in the resulting PDF file came out at 200 dpi instead. The Grayscale image came out at 200 dpi as well.
Thanks! Hope it gets fixed soon.
This bug is here since CS3! For 12 years! Are you serious?!
Mark R commented
This wasn't caught in the latest beta Illustrator CC 2019 update, so best bet, for now, is exporting PDFs without image resampling and do all your resampling in Photoshop. Luckily this bug isn't present in Indesign, that would be a nightmare. Hope some more people notice it, then it may actually get fixed. A couple other bugs regarding monochrome bitmaps were fixed in the latest beta, I recommend installing it if you work with bitmap line-art.
Thought I was going mad trying to export a print PDF with some 1200 ppi bitmap lineart. When examining it in Acrobat Print producing getting an error that it had been downsampled. It maintains the bitmap but takes the resolution down to under 500 ppi for me. Thanks for reporting this and hopefully it gets fixed soon!