Welcome @Pietrogramma,
Sounds like you want to use Nextcloud folder as the source and a mount point on the HDDs as the destination?
I don’t use Nextcloud or Duplicati on a NAS but from what I can see Nextcloud uses the local file system on the NAS so you should be able to just point Duplicati to the Nextcloud folder as it’s source then specify the mount point for the HDDs as the destination. This would be a “Local folder or drive” type of backup job.
As far as I know, Duplicati cannot selectively backup only the .doc, .xls and .pdf files in a folder. You could create filters and exclude all the other file types in the folder but that’s not a very clean option. EDIT: Turns out you can selectively backup just the *.whatever files in a folder, see this thread for details.
I also found this article on backing up Nextcloud, in the article they create a backup directly from Nextcloud (using nextcloud.export) then backup that file to other locations using Duplicati. Not quite what you want but maybe of some use.
I hope some of this helps.