This refers just to online storage of attachments. No need to remove the files -- you can just disable file sync in the Zotero preferences.

Those are two different things, though: the PDF files stored locally and saved with all items do not count against your storage quota unless you have file sync enabled. If you find them useful to have locally (as most people do), I'd recommend leaving that preference enabled.

Luca I regularly remove PDFs despite having large storage volumina.

The procedure for dealing (removing) with PDFs is the following.

Part I: Decide if you want to retrieve PDFs in first place when adding new items. If you don't want the provided PDFs, deselect [ ] Automatically attach associated PDFs and other files when saving items in Preferences->General->File Handling section

Part II: If you want to remove PDFs from Zotero from within the app, do the following.

Step 1: Create a advanced filter using "Advanced Search" and Match all "Attachment File Type" is PDF. You will see a list of items highlighting the PDFs only, not the parent item (article, book, etc.). You cannot delete inside of the search window, you must "Save Search" (as a filter)

Step 2) Go into the main window of Zotero app. Find the filter on the side in the lower bottom, near Trash.

Step 3) Select the filter, which re-produces the same results as the filter window did. There, select all and delete (DEL). This will move the attachments only to Trash.

Step 4) Check Trash to be sure everything is Ok. Check your list of items once again, that the main item is still available, but the attachment was remove

Step 5) Finally, empty the Trash

WARNING: Try with one example first, before running through all your library.