Using fax viewer software.
Inbound faxes are normally delivered to you by email, with the fax attached as a file. The file format of the attachment will usually be TIF, but some providers will give you the option of receiving it as PDF, JPG, or other formats.
Other providers (usually providers of free services) may require a proprietary viewer. This means that you can only view their faxes through their software, which you install on your computer, and which will sometimes include forms of advertising.
Windows itself includes built-in viewer software, in its recent versions, so double-clicking on the attachment will automatically open the Windows Fax Viewer. This program will let you move between pages of a fax, zoom in and out, or even edit the contents of a fax with Window's Fax Editor.
It is recommended not to use graphics software, which was not intended for the purpose of viewing faxes, such as Paint Shop Pro or ACDSee, because these programs can neither handle the awkward resolutions of some faxes (200 dpi width-wise and 100 dpi length-wise), nor display any pages beyond the first page of a fax.