You can get to opera:plugins in Opera Mail by editing help/en/en.html in the opera program files directory and adding

<meta http-equiv="refresh" content="0; url=opera:plugins">

to the head. Then, you just press F1.

But, you shouldn't need to do that as the pdf attachment isn't going to open in a new tab in Opera Mail and get embedded with the adobe reader plugin. It should just launch with the default application.

Now, you could put

[File Types Section Info] Version=3 [Trusted Protocols Section Info] Version=1

in Opera Mail's operaprefs.ini (in your preferences directory) and add handlers.ini with

Opera Preferences version 2.1 ; Do not edit this file while Opera is running ; This file is stored in UTF-8 encoding [application/pdf] Type=Viewer Action=4 Application= Application Description Web handler Plugin Path=C:\Program Files (x86)\Adobe\Acrobat Reader DC\Reader\Browser\nppdf32.dll Plugin Name=Adobe Acrobat Plugin Description=Adobe PDF Plug-In For Firefox and Netscape 17.9.20044 Save To Folder= Extension=pdf Flags=128

in the preferences directory and start Opera. (Flags are set to have application pdf open with the default application. You can play with an Opera 12 setup's file type settings and then look at handlers.ini for changes.)

But, you shouldn't have to do that either. Things work fine for me with pdf attachments on Win10 with Opera mail and Adobe.

I would goto Control Panel\Programs\Default Programs\Set Default Programs in Windows, select Adobe and make sure it has all its defaults. I would skip the new Win 10 default apps settings.