There may be times when you need to add an item to the library by hand. If so, use the green "plus" icon in the desktop version and choose what type of item you are adding from the drop-down list. A window will open with the appropriate fields to complete for that item type.
If you have the book or article in front of you, you can add it to your Zotero library easily without having to search for a citation.
If you have a book's ISBN, an article's DOI or PMID number, just click the magic wand button (Add item by identifier) above your list of items. Type in the number, and Zotero will automatically search for the appropriate information, download it, and save it to your library.
If you have set up your Zotero preferences to automatically save PDFs and snapshots, then they will be added as you add citations. Attachments can take up a lot of space and may quickly overload a free account. You can purchase more space if you want to keep everything together.
Files can be attached manually by dragging a PDF (or other filetype) into your Zotero desktop. Dropping a file into a collection or in between library items will copy it into your library as a standalone item. Dropping it onto an existing item will attach it to that item.
Each item also has an Attachments tab in the right column. This allows you to attach files by using an Add button.
If you have a host of PDFs already on your computer, you can easily add them to your Zotero library and have Zotero extract their citation information.
Adding citations to your Zotero is easy from within databases, Google Scholar, or just general web searches. You'll need to do the following to be able to do so:
If Zotero detects that you're looking at a book or article, you will see a book or page icon appear in or next to the address bar of your browser. Just click on the icon and Zotero will automatically save the citation and either download the PDF if it's available or take a snapshot of the page.
If you're on a page of search results, like you would see in a database or using Google Scholar, you'll see a folder icon instead. Clicking on the folder will show you a list of items on the page. You can then check off the ones you would like to save.
There are some websites from which Zotero simply cannot capture the citation information. Generally, those are pages that lack metadata which defines the author, title, etc. You can still have Zotero add them, and it will take a snapshot of the page, but you'll have to edit the entry.
To add it, right-click in your browser and choose "Save to Zotero" from the menu. This will save a new "web page" item to your library.
A snapshot is a copy of the page that is saved to your computer. It includes the text and images so if the page is removed later you will still be able to view your copy of it.