If you need to open CAD files in multiple formats and can't afford to pay for AutoCAD, then AutoDWG DWGSee will do the trick. AutoDWG DWGSee allows you to view CAD files in multiple formats without having to install the expensive editor. Not only does it allow you to view them, but you can also use the program to navigate CAD files and folders and print them. The Pro version even lets you convert them to image formats and PDF.
AutoDWG DWGSee offers a decent range of tools, including support for several ways of navigation and visualization, as well as the ability to add manifold marking and layers and even print the file or just a part of it. The Pro version allows you to convert from DWG, DXF and DWF to a range of graphical formats like TIFF, JPG, GIF or even PDF. On the downside, the program only opens one file at a time.
Although the program won't actually let you edit CAD files, you can use AutoDWG DWGSee to measure areas within files, making it a great way for reviewing designs.