The tool generates print ready vector PDF files (to X4 standard). You should not need to edit them, but if you do they can be opened and edited in Adobe Illustrator, the industry standard software for editing such files.
Our PDF's are designed to be 'Placed' into the Illustrator artwork as a link. There is a short video demonstrating this here:
https://help.oprl.org.uk/hubfs/Label-Tool-Instructions/OPRL-Label-Tool_Placing-label-artwork.mp4
Having done that, it's also possible to embed the label in the artwork and convert the text to outlines at the same time. There is a video on that here: