Thanks Martin. Chrome is usually is the least problematic browser.
Some more information on this... and it is rather strange!
If I use the print (to PDF) feature in Edge, the coloured backgrounds of the lables are ignored.
If I use the print (to PDF) feature in Chrome, the coloured backgrounds of the lables are reproduced correctly.
This appears to be an attribute of the PDF produced not the browser used to display it. (Tested by cross loading the PDFs to the browser they were not produced with.)
Thanks for the feedback Martyn. I'm not sure how easy this would be to change, but as an alternative try using the Print Using Browser option and use your browser's PDF export facility - this works OK in Chrome.
When I create a route, I sometimes use the feature for selecting a colour for a lable. I use this to distinguish different categories of label. For example, I might use a one colour for information lables (e.g., for a cafe) and a different colour for labels necessary to route finding.
The colours show correctly on screen but are ignored when using the print feature to produce a PDF.