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.