Zotero is free to use. The only thing you may choose to pay for is how whether you want attachment files (like pdfs) to be uploaded to Zotero's cloud and synced between your computers. You can still attach as many pdfs locally in your computer, read and work with them - they just don't sync between computers (there are some alternatives to Zotero's storage, but not as convenient and functional).

Otherwise, all your saved data are freely uploaded and synced. And there is no limit to how much data you can sync.