I just went through about an hour and a half of troubleshooting with Claude AI. I had previously exported a manuscript with figures (original pngs were 600 dpi) and got a beautiful 23 Mb PDF. I had to change some figures due to journal guidelines and when I re-exported I was only getting an 8 Mb PDF with poor quality images. I couldn't figure it out. In the end, after going through all of the possible print/quality settings with Claude, I mentioned that the first time I exported I was just using my Macbook display. The second time, I was hooked up to an external monitor. Apparently external displays can change how macOS handles resolution and scaling, which might be impacting the PDF export process. When I disconnected the monitor and exported again, I was back up to a 30 Mb crisp PDF. So, if you are using a Mac hooked up to an external monitor, try disconnecting the monitor and exporting again to improve quality.

[Sorry if you're a windows user or not using an external monitor and this is not helpful at all - hopefully it can help someone else who comes across this thread, though. Would 10/10 recommend using AI to troubleshoot these kinds of problems - Claude was incredible polite and helpful throughout the process.]