At the URL opera://settings/content/pdfDocuments, you have the default of "Open PDFs in Opera" where the PDF file will load in Opera with the built-in Chromium PDF viewer. Sounds like something is broken with your install if the Chromium PDF Viewer isn't working. I'd try reinstalling Opera for good measure. If that doesn't work, and the link to the PDF file in your email points to a non-secure page, while on your webmail, you might have to click the badge at the left of the address field, goto "site settings", and allow insecure content.
If you change the setting to "Download PDFs", Opera will just download the pdf file where you can then open the file to open it in the default PDF program on your system (which can be Adobe if you set it as the default for PDF files). There's no option to automatically open the pdf file on your system once it's downloaded, and there's no option to not download the PDF and just pass the URL to the PDF to the default PDF program for it to download it.