No need for plugin, calibre now has a built in Open With.
Right click on a PDF in Book Details, select
Open PDF With, if your program isn't listed, then select
Add other application for PDF Files... and browse to program.
An entry will be added to
Preferences->Keyboard->Open With for the program which you can make use of - e.g. if you selected Nitro PDF Editor you could give it a keyboard shortcut of Ctrl+Shift+N.
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