When selecting an item in Zotero, you can make a note about it on the Notes tab of the right-side pane.
Notes, as well as other metadata, are searchable in Zotero's search box.
As of Zotero 6 (March 2022), Zotero's built-in PDF viewer will allow you to make annotations like highlighting and notes directly to the PDF document and save those annotations in cloud storage so you can access them on any computer that has Zotero installed. Read more about the Zotero 6 update on Zotero's blog.
Tags are a powerful tool in Zotero helping you to further group and categorize your sources. Many sources are imported from academic databases with a number of default tags based on the subject of the article. You can also create your own custom tags. Tags for each source are managed on the Tags tab in the right-side pane of the Zotero application.
The bottom-left corner of Zotero is further dedicated to searching and managing of tags. You can recolor tags to help color code sources for your convenience. For example, some users use colored tags to indicate articles that are read or unread.
Sometimes you might end up adding the same source into Zotero more than once. You can manually delete an item by right-clicking and selecting Move Item to Trash, but Zotero also has a Duplicate Items option on the left-hand pane to help identify duplicates across all of your folders. You can use this feature to identify duplicate items and merge them together.
Many full-text documents from ACU's OneSearch import into Zotero with a filename such as "EBSCO Full Text." To apply more descriptive file names, right-click on the PDF and click on "Rename File from Parent Metadata." This will rename the file using the article's title, author, and year.