Output doesn't export to PDF (grayed out)
Although all files are selected and every setting is filled in, the knob 'Export to PDF' stays unselectable (grey)
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Dominique, I hope this has been resolved for you since 2018 :-)
If you are using 'Output' to create a contact sheet from Bridge...
It's easy to forget the step of dragging the selected images onto the visual grid displayed in the interface. (I get it -- usually when you already have the files selected that is sufficient to pass along to the next step.)
Upon dragging images to the grid, the page range is calculated and Export becomes immediately available.
Same issue with Adobe Bridge 2018 and 2019. Not all of the time, but often enough to be a pain. And even after option-clicking to start the app and deleting and resetting preferences and cache. No change.