How can I specify the order files should be compressed in 7zip?
I managed to solve this problem. The solution was to create an archive containing only the miscellaneous files, and after that select "add to archive" from the explorer context menu while selecting both the PDF and JPGs. In the 7zip "Add to Archive" dialog, I chose the same compression settings and archive name as before.
This compressed the PDF and JPGs together - taking full advantage of their redundancy - then added them to the existing archive. It resulted in an overall 45% compression ratio, exactly what I was looking for.