O.K. I've done a bit more investigation.
Tried installing Chromium on Windows 10 and everything works well. I don't get the scale option but it came out right first time both on my HP printer and on Microsoft PDF. I think the scale option might depend on the printer connected.
Tried Firefox (still at v;72) on W10 and everything worked although the reader did crash once. However Firefox updated itself to v.73.0.1 and the reader no longer displays the magazine contents so I've reported that to Pocketmags support.
So, if anyone else is using Firefox on Windows or Linux they would be advised to avoid updating to v.73.0.x if possible and if they wish to use pocketmags on it.
I also tried installing the Windows Pocketmags app. It works quite well except that there is only one level of zoom available and no print option as far as I could tell.
Russell