Consider Disabling Automatic Attachments and Tags
Attachments are files that are associated with the items in your library. This includes notes, webpage snapshots, and full-text files. This is what an item with attachments looks like:
The default setting is for Zotero to add attachments automatically, such as a webpage snapshot and the full text PDF if available. This can be convenient, but if you want to save on Zotero server storage space then you should consider disabling this feature. You can still selectively choose when to attach a PDF by adding attachments manually.
Tags allow you to categorize and sort items by associating relevant keywords with items in your library. If you are importing an item from a library catalog or database that includes subject classification information, Zotero will detect this and import these classifications as tags. Many people prefer to set up customized invidual tags rather than using library subject classifications when sorting through their Zotero libraries. Consider disabling automatic tags to keep your library clutter free.
You can disable automatic attachments and tags by clicking on the gear icon, choosing Preferences, and clicking on the General tab of the Preferences menu. Under the Miscellaneous heading, uncheck "Automatically take snapshots when creating items from web pages", "Automatically attach associated PDFs and other files when saving items", and Automatically tag items with keywords and subject headings."