Come to find out Xubuntu 18.04 ratcheted the ImageMagick security settings up to a dangerous chattering whine:
convert p???.jpg "Machining D-bit Drills.pdf" convert-im6.q16: not authorized `Machining D-bit Drills.pdf' @ error/constitute.c/WriteImage/1037.
Fortunately, someone who understands this stuff encountered the problem before I did and posted a great description of the solution.
To forestall link rot, the process looks like:
cd /etc/ImageMagick-6/ sudo cp policy.xml policy.xml.base sudo nano policy.xml … change one line … policy domain="coder" rights="read|write" pattern="PDF"
It is completely unclear to me whether ImageMagick (as of ImageMagick 6.9.7-4 Q16 x86_64 20170114 ) requires or merely tolerates the vertical bar in place of commas, nor whether it’s in my best interest to replace
In any event, I can once again stuff bitmap images into PDF files.