You can add sources to your library in Zotero in several different ways.

From within Zotero, click File and then New Item to manually create a new source entry in your library.

Along the top of the Zotero desktop application, you can also click the magic wand icon to Add Item(s) by Identifier. Here, you can pull in metadata for a source by entering its ISBN, DOI, PubMed ID, arXiv ID, or ADS Bibcode.

You can also import a file into Zotero by going to File and then Import. This will enable you to import a BibTex, RIS, Zotero RDF, or even to import your library from Mendeley Reference Manager.

You can also add sources to your Mendeley library using the Zotero Connector, a browser extension that is available for Chrome, Firefox, Safari and Edge.

The Zotero Connector can detect sources that are on a webpage that you are viewing and then you can select which ones you want to add to your library, or to a specific collection within your library. You can even save the webpage itself to your library or to a specific collection.

In the above screenshot, an article on Pitchfork has been saved to the collection test_group and Zotero has also taken a snapshot of the page that will be saved in the Zotero library.

In the above screenshot, a search in MRU LibrarySearch for pop music has retrieved a list of results, and the user has used the Zotero Connector to detect them and then to add a specific one about K-pop.

Both sources can then be seen in the collection test_group on the Zotero desktop application and their metadata can be edited using the right-hand side panel.

You will notice that Zotero did not add a PDF file for the K-pop article, because it does not have a dot beside it under the paperclip column in the desktop application. If you wanted to add the PDF file to the source, you could download the article and then drag it on to the reference in Zotero to link the two and save them together.

If you want to save the PDF directly into Zotero, it tends to work best to navigate to the actual PDF within your browser and then use the Zotero Connector to save it to a library or collection. This will then usually save the reference and the PDF to the library.

More information on adding references to Zotero can be found here.