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Do so by opening the red Adobe Reader app with the stylized, white A icon. Then click on File in the menu bar at the top of the screen, click on Open..., select the PDF document you want to type on and click on Open. If you don't already have Adobe Reader, it's available for free from get.adobe.com/reader and can be used with Windows, Mac, and Android operating systems. It's the marker icon on the right side of the toolbar at the top of the window.   The text is now highlighted. Doing so saves your highlighting.

Summary:
Open a PDF document in Adobe Reader. Click on the highlighter tool. Position the cursor at the beginning of the text you want to highlight. Click and hold, then drag the cursor across the text. Release the click when you are done. Click on File in the menu bar and Save in the drop-down menu.