There are a number of ways to add files to your Zotero Library. One easy option is simply to drag and drop the files into the Zotero App and the Library of your choice.
But if you need to adding multiple files at a time then using the "Link to File" or "Story Copy of File" featured in the drop down menu under the green plus sign "New Item" icon are great tools to be aware of as well. The tutorial below outlines how to use each and their different roles in adding files to your Zotero Libraries.
Once you have files in your Zotero Library, there are a few different ways you can create Item Records for those files. You'll want to do this so that Zotero is able to cite these documents when you get to that stage in your work.
The most common way is by right clicking the item you need to create the Item Record for and choosing the option "Create Parent Item." The tutorial below outlines the steps to using this feature to create the Item Record.
If you have any PDFs, you may be able to easily create an Item Record from the metadata in the PDF. This isn't full proof but it's a nice option to have.
To get the menu below, simply select and right-click the PDF that you want to create the Item Record for and select "Retrieve Metadata from PDF"