You can join multiple images to pdf using `convert`:
convert *.png out.pdf
If you get an error like
convert-im6.q16: attempt to perform an operation not allowed by the security policy `PDF' @ error/constitute.c/IsCoderAuthorized/408.
follow the steps below:
sudo vim /etc/ImageMagick-6/policy.xml
and replace the line
with
[source](https://askubuntu.com/questions/1081895/trouble-with-batch-conversion-of-png-to-pdf-using-convert)