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If the font is available from a website (such as Google Fonts), simply select the font, along with its font weights and insert the generated @import declaration in your CSS:
@import url('https://fonts.googleapis.com/css?family=Montserrat:300,300i,400,500,700i');

section h2 {
    font-family: Montserrat, Serif;
    font-weight:500;
}
Tip: It is possible to add a fallback to the remote font (Serif in the above example).
Important: Make sure the website hosting the font is accessible. If you get errors in the console (for example, Unknown host or I/O Exception) regarding one of the font resources, check your networking proxy settings or your firewall settings. For the parameters that control the HTTP proxy, see Generating PDF Output from XML or HTML Content Using a Command Line.