Adobe Illustrator files usually have the file extension .ai. If you have Adobe Illustrator, it's the best tool to use as a .ai file viewer or editor. If you don't have the software, you can open Adobe Illustrator files in other ways.
Open Adobe Illustrator Files
Adobe Illustrator is a common commercial tool used by artists and graphic designers. Adobe Illustrator files are vector graphics files, meaning they specify graphics using the mathematical relationships between lines and points and other objects. Designers sometimes send them to clients and colleagues who also use Illustrator, but even if you don't have the software, you can open them with a number of common tools.
One technique is to rename the files to change the extension to PDF, another file format maintained by Adobe. Adobe's Adobe Reader software, which is available free for most platforms, can then open the file. You won't be able to edit the files in Adobe Reader, but you can view them.
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You can also preview Illustrator files using some cloud storage tools, including Dropbox.
Other Vector Graphics Software
Other vector graphics software can function as an Illustrator file editor. You can download a free tool called Inkscape for Windows or macOS that edits the files and transforms them to other vector graphics formats, such as the SVG format commonly used on the web.
CorelDraw, another popular graphics program, can also open Adobe Illustrator files and export them in other formats.
Cloud Conversion Tools
Online tools, such as CloudConvert and Convertio, can convert files from one format to another. They import .ai files and output them in other graphics formats such as SVG, EPS or as other types of image files.
Depending on the software you work with and how you intend to use the graphic, one format might be more useful to you than another.
Adobe Illustrator Free Trial
If you don't anticipate needing Adobe Illustrator in the long run, try a free seven-day trial of Illustrator from Adobe. Depending on your needs, the free trial version might be sufficient to open a particular file and do the work that you need to do before you have to pay for the software.
Check Adobe's website for all the details. Also, check with your public library. Some public libraries provide free access to Illustrator on computers in the library.
Have the Sender Convert It
If someone sent you a .ai file not realizing you don't have Adobe Illustrator, ask the person to export it in a different format.
PDF is useful if you only need to view it, and SVG is useful if you need to make changes to the design in another vector graphics tool. EPS is useful in many other circumstances.