You can add many different items to your Zotero page, such as webpages, Word documents, PDFs, RIS/BIB files, jpegs, and more!  If you want to add webpages, just make sure that the Zotero extension is added to your browser.  In addition to the boxes below, you can also click on this link to get more information about adding to your Zotero library.,

Once you have enabled the Zotero extension and pinned it to your toolbar, you will have the option of saving webpages to your Zotero library.  You will see an icon that looks like a puzzle piece, and to the left of that, you will see a save button that is blue when you are able to save the webpage to your Zotero library. 

You can create a specific folder for these in Zotero, or you can simply save them and organize them later.

You can save references and upload materials via the plus sign at the top of the Zotero Library page.  You can do this from a folder or from the main page.  Once you click on the plus sign, you have several options, including:  Book, Book Section, Case, Hearing, Journal Article, Upload File, and More.  Select the appropriate option and fill in the requested information.

To add a PDF from your desktop, you would use the "Upload File" button.  See the screenshot below:

Select the file from the appropriate folder on your computer.  Then, click on "Open."

You should now see your document on the Zotero page.  The document selected above is showing on the Zotero page below.

For this feature, you will need to have the Zotero extension enabled in your browser.  This feature for adding RIS and BIB files is particularly good when you are working with databases and want to import either a few or whole batches of search results.  You will be using the icon that looks like a pin.  Click on the pin, and you will notice that it gets a little cross out to let you know that you have selected this item.

The item will now appear in your "favorites," which you can find in the tool bar in the top right corner.

When you click on this pin at the top of the page, it takes you to your favorites, and you can select some or all the items that you have in your favorites.  For all of them, click the top box:

Once you have selected the item or items that you desire to save and export, you will click on the three dots on the My Favorites toolbar:

You will have a number of choices available.  Click on Export RIS:

Then, click on Download:

Depending on the browser you are using and the computer that you are using, when you click on "Download," you may get different options for where to save this document.  For this example, Chrome is being used on a PC.  So, if you are using Chrome and a PC, you will most likely see the option below, and you can choose where you wish to save the file.

For the above example, the document was saved to the desktop for easy access.  Now, open up Zotero from your desktop and click on "File" (left-hand corner), and select "Import."

A box will pop up, and you will notice that "RIS" is one of the files listed.  You will click on "Next."

Select your RIS from from the place where you saved it.  In the example below, the RIS file was saved on the desktop.  Once you have selected the file, click on "Open."

The option "Copy File to the Zotero Storage Folder" should already be selected, and then click "Next."

Once you click on "Next," then you will click on "Finish."

You will then be able to see the item in both the Desktop and online versions of Zotero.  The screenshot below is from the online version: