After opening a new document the first thing you will want to do is set up your work area. To change the size of the artboard choose File>Document Setup. The Document Setup dialogue box will appear.You can use the dialogue box to choose a preset artboard size from the Size pop up menu. Or you can create a custom size artboard as large as 227 inches by 227 inches.

The Units dialogue box allows you to select among Points, Picas, Inches, Millimeters, Centimeters, and Pixels. One inch equals 72 points. You are probably familiar with points from font sizes. Picas are 12 points or 1/6 of an inch. Pixels are relative values tied to the display resolution on a given system but are roughly equal to points.

If you want to change the units in the document choose Illustrator>Preferences>Units & Display Performance.

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You can also open up saved documents into Illustrator. Choose File>Open to open the document as a new Adobe Illustrator Document. (The file extension for Illustrator documents is document_name.ai)

Note: You can open PDF files and edit them in Illustrator.