What do you mean by "uploaded"? Files are indeed only uploaded to the Zotero servers if file syncing is enabled. But Zotero will still store files within the Zotero data directory by default. If you don't want that, you'd have to add them as linked files.

(Also, I'm not sure what you mean by "I do not wish to have pdf copies of my research items in my Zotero library". If you're using Zotero, you want the attachments to be in your library so you can open them. Whether they're stored in the data directory is a separate question.)

You can use Zotero without copying files into it, yes (uploading is a completely separate topic: you can use Zotero without uploading anything ever; it's principally a local application).

To extract metadata from existing PDFs, they do need to be somehow linked to Zotero, but you don't need to store copies of those files with Zotero to do this. You can just link to them in their current location (e.g. on your OneDrive folder). See https://www.zotero.org/support/attaching_files#stored_files_and_linked_files for that distinction and https://www.zotero.org/support/attaching_files#adding_linked_files for how to add files as linked files instead of storing a copy in Zotero.