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 

    <policy domain="coder" rights="none" pattern="PDF" />

with

    <policy domain="coder" rights="read|write" pattern="PDF" />


[source](https://askubuntu.com/questions/1081895/trouble-with-batch-conversion-of-png-to-pdf-using-convert)