Where were you getting these Opera opening links from?
It is possible to include information in a link that specifies a specific app. The feature was intended for internal links, like linking to a file in GoodReader; but it's been uses for other things since.
There were two specific emails from European companies, each containing attachments in .pdf format. I couldn't open them in Adobe Reader from Opera.
All sorted now and I don't expect any further communication from either company. Having deleted and re-installed Opera everything seems to be behaving as required.