If you have an ISBN, DOI or Pub Med ID, then Zotero can look up its citation information
The item is very easily identified with all the information that is available
The citation information that Zotero captures is usually correct. Nevertheless it is always recommended that you double check every item's metadata for accuracy. It is also written in Zotero's website that "Zotero will accurately import the metadata but the metadata may not be accurate".
Fields can be edited by clicking on the field. A blank box will appear and you can type in additional information.
Zotero can import bibliographic data in various forms, including PDF. The easiest way to add a PDF document from your computer in your library is to drag and add it into your library. You can retrieve metadata for these PDFs by right clicking on each one of them.
If there is metadata, Zotero will automatically present it at the right tab. If no metadata is found, you should manually fill out the fields.