If you're using a Safari Browser and are unable to view or download the PDF work order (or any PDF file), chances are an old PDF plugin is the culprit.
There was a time when a special plugin was required to view PDF's in Safari, but that is no longer the case on this functionality is natively built into the browser these days. The solution is simple: Locate the old plug-in in your Library/Internet Plug-Ins folder and uninstall it.
There appears to be multiple PDF viewing plug-in's that could be responsible for this problem. According to Adobe, their plug-in is named "Adobe PDFViewer.plugin". Another Adobe plug-in is named "NPAPI". Additional instructions can be found on this tech blog.