How to change the document font in LaTeX

This could be done via CSS:

<style type="text/css">
@font-face {
    font-family: "My Custom Font";
    src: url(http://www.example.org/mycustomfont.ttf) format("truetype");
}
p.customfont { 
    font-family: "My Custom Font", Verdana, Tahoma;
}
</style>
<p class="customfont">Hello world!</p>

It is supported for all of the regular browsers if you use TrueType-Fonts (TTF), the Web Open Font Format (WOFF) or Embedded Opentype (EOT).

You can use the sectsty LaTeX package.

Put this in the preamble:

\usepackage{sectsty}
\allsectionsfont{\sffamily}