I agree with Ryan, I'd just use some CSS or JS to hide the button as needed. With some additional logic you could make the PDF button hide and show as needed depending on variables as required.
The CSS solution doesn't work for us as I'm finding that the ID changes every time the form loads...
1st - Nintex.Forms.SharePoint.Ribbon.FillerGroup.ca3dacb75ac5462f9be8a6e58e19377f
2nd - Nintex.Forms.SharePoint.Ribbon.FillerGroup.acd997ca69114adfadc1465498e9664a
Is anyone else having this issue?
I cannot confirm this issue. I tested it for the cancel button in preview and in published version, the id of the button always remains the same (even after re-publishing the form). Which control did you try to hide?