Posting this as an FYI to others - if the documents you create with MultiMerge::mergeDocx cannot subsequently be converted to pdf with libreoffice, check your libreoffice version. We did not have a problem with transformDocument on any of the environments; this was only a problem for documents concatenated with mergeDocx. We were able to confirm it was not a permissions issue.
Using phpdocx 8.5 in both environments, we had no problem doing this on OSX, but it was failing on our linux environment. We found the problem was due to using libreoffice 6.x on the macs, and 5.x on linux. Upgrading to libreoffice 6 fixed the problem.