you can take a look at the conversion program called 'PaperCrop'.
https://www.mobileread.com/forums/showthread.php?t=31677
It can process quite complex PDFs into smaller images and it also supports reflow. It lacks some features such as pre-trim, but this can be a really valuable tool for reading PDFs on 6 inch device. Since the program creates image or PDF files, it can be useful for any device. You can try this tool and see whether it alleviates the problem of reading PDFs in 6 inch e-ink reader.
However, I would agree with others that having letter/A4 size e-ink device will be really nice for reading PDFs.