In a PDF article, look for the search icon (it will probably be a little magnifying glass). Then, type a word or phrase that you'd like to find in the article's text. The first match will be highlighted, and you can use the arrows to jump to the next or previous matches. Or, select "highlight all" to see where all of the matches are at once.
Here's an example from an article in an EBSCO database (with "highlight all" selected):
In an HTML article, the process will vary a bit, depending on the browser or operating system you are using.
If you are using a PC, you can press "Ctrl-F" on your keyboard to launch a search box.
On a Mac, use "Command-F" or "⌘-F" instead.
A search box should appear in your browser window, either at the top of the bottom of the page. Type the word you'd like to search for, and it should be highlighted on the page. Here's an example from the Chrome browser: