You can use Link to File to create linked files from the original or use the ZotFile plugin to create linked files elsewhere, but Zotero uses stored files by default, and that's what we recommend, since it provides the most seamless, least error-prone experience. The normal way of using Zotero is to save items from the web and let Zotero download and manage PDFs for you.
Thanks! Some users may perfer not saving into certain path. What's more, when the link to file(PDF file, a paper) is added to zotero, it can't show information of paper.
What's more, when the link to file(PDF file, a paper) is added to zotero, it can't show information of paper.
No, this is a misunderstanding. Whether you use stored or linked files makes no difference in terms of metadata. In either case Zotero will attempt to retrieve metadata for the PDF, and if it can't, you'll need to create a parent item, as explained on that page.
Saving from article pages on the web, as we recommend, avoids the need to try to retrieve metadata and will attach the PDF to the metadata item if the PDF is available (which, in relation to your other thread, is one benefit of doing it in the browser, where you might have proxy-based access to the file, which the Zotero app itself doesn't have).
“if it can't, you'll need to create a parent item, as explained on that page.” : That's what I happened,a link file can't create retrieve metadata, while parent item is ok.
No, again, you're misunderstanding this. There's no difference between a stored file and a linked file in terms of metadata, so it has nothing to do with your question. Either way you need a parent item. PDF metadata retrieval works exactly the same on either type of attachment. Whether metadata can be retrieved depends entirely on the PDF in question, not how it's stored.
Upgrade Storage
https://www.zotero.org/support/attaching_files#stored_files_and_linked_files
Saving from article pages on the web, as we recommend, avoids the need to try to retrieve metadata and will attach the PDF to the metadata item if the PDF is available (which, in relation to your other thread, is one benefit of doing it in the browser, where you might have proxy-based access to the file, which the Zotero app itself doesn't have).