When you try to convert multiple image files into a single PDF, you may see a series of errors that look like this:

convert-im6.q16: cache resources exhausted `file1.jpg' @ error/cache.c/OpenPixelCache/4095.

You can solve this by increasing the amount of memory available to Imagemagick. First, locate the Imagemagick policy file. It will be something like /etc/ImageMagick-6/policy.xml. Then, find a line that looks like this:

<policy domain="resource" name="disk" value="1GiB"/>

And increase the value, e.g.:

<policy domain="resource" name="disk" value="8GiB"/>