The create .pdf option is greyed out when there are no model views selected. Depending on your setting for model view load options (default is by node) there may not be model views available to produce .pdf output from at the node you have selected in the product structure tree.
One option is to turn on the Model View Snapshots, as you navigate the tree the snapshots will update with the model views available . The create pdf option will output the model views visible in the snapshot gallery.
I have JT2GO 11.4.0 and I don't see Create PDF with JT at all in the JT2GO->Attachments menu.
I have Model Views, thumbnails etc. Why am I not seeing the "Create PDF with JT" button?