How to bulk convert .djvu files to .pdf?

You could use ddjvu, in a shell script. That said, the output PDFs are much larger (x10), which makes it hardly worth the effort. Ubuntu has no problem reading djvu files, but if your reason is good enough, use the following script.

Warning: Do not try it on 200 files right away. Run a test first on one or two small ones, to get the feel of how long it takes, and to make sure you are satisfied with the result. Press Ctrl + C, in case you want to stop the process.

#!/bin/bash
for i in *.djvu;
do ddjvu -format=pdf -scale=100 "$i" "${i/%.djvu/}.pdf"
done

...or the same, as a one-liner

for i in *.djvu; do ddjvu -format=pdf -scale=100 "$i" "${i/%.djvu/}.pdf";done

Simply run that in a folder with djvu files. The -scale=100 option downscales the output images, which makes the process much faster, and the output files' size more reasonable. Without it, the resulting PDFs were much larger then the originals, and took ages to convert, at least in my tests.