Zotero automatically downloads the pdf full text if available or takes a snapshot of a webpage. Use this option if the pdf file was not downloaded or you added a source manually. If you have a pdf file but haven't created a source record, you can drag and drop the pdf into the Library or a collection and Zotero will automatically create the record.
Note: If you are syncing with your online account, attached files take up significant space. To avoid running out of space, uncheck the sync full-text content to prevent files from being uploaded to your online account.