It's been noticed that sometimes the task can get stuck for hours and even days. Last time when I was investigating this the task had been running for 2 days. And it was actually running. Here is its trace:

root@220d00d79f97:/# lsof -p 24541

COMMAND   PID     USER   FD   TYPE             DEVICE  SIZE/OFF                NODE NAME

php     24541 www-data  cwd    DIR               0,73      4096             1726998 /var/www/site/admin/cli

...

php     24541 www-data   14r   REG               0,54 642457217 9881900796078757721 /var/lib/sitedata/temp/assignfeedback_editpdf/pageimages/826142dc145f485b29f562f94d1d162e00f3f0ab/combined.pdf

php     24541 www-data   15r  FIFO               0,13       0t0           260879137 pipe

The file the task was working on was really large: 613 MB (/var/lib/sitedata/temp/assignfeedback_editpdf/pageimages/826142dc145f485b29f562f94d1d162e00f3f0ab/combined.pdf).

After some time I found the original submission. This submission has two files: PDF and RMD. If I'm getting this right both are not supposed to be converted by the task.

Interesting that PDF file had only 14 pages in it, but its size was over 100MB. It seems like this PDF was exported from some math software and it had lots of data sets for graphs in it.