I created an index to a folder on my NAS. Long as I am connected, it is convenient. Is there a way to continually sync the .pdf files (or whatever) to the folder that was imported??? Or is there a different/better way? 
I’m not certain I understand what you want to do, but let’s try this scenario:
You have Index-captured a folder that’s on an external server. The result is that your database now holds a corresponding group that contains the set of documents corresponding to the files contained in that Index-captured folder.
As long as the server remains mounted, so that the Paths of the Indexed items are functional, you can select a certain PDF and view, print or even edit it and save the changes.
But if you unmount the server, the Paths are broken. That certain PDF can no longer be displayed when selected in your database.
But that file is important and you want to be able to view it when the server isn’t mounted. For example, your database is on your laptop and you want to be able to access that PDF while you are on travel.
You cam select that PDF while the server is mounted, Control-click on the PDF and choose the contextual menu option, Move to Database. That PDF has been copied into the database (Imported), and although it remains filed inside the Indexed group, it is no longer Indexed and will be accessible while you are on travel.
When you return, perhaps you may want to make that certain PFD Indexed once more. With the server mounted, Control-click on the PDF and choose the contextual menu option, Move to External Folder. That PDF is now Indexed once more (but if and only if it was in an Indexed group when Control-clicked - if not, that contextual menu option will not be available).
But remember that during the time that certain PDF was Imported into your database, there is no communication between it and the external copy in the Finder. I’ll leave it to you to experiment. If you had edited both copies of the PDF independently while they were separate entities, which edit will be retained if you choose the Move to External Folder command? Suppose you had, while on travel, used that PDF’s Item LInk to link to it from a note. Will the link still function after the PDF was moved to the External Folder? What about a tag applied to it while on travel?
Thanks, I will review my requirements for this.