My organisation uses a few fairly standard fonts that aren't available in Civi, for mailing labels and letters and auto-response emails etc. - how can they be added?
They are several places where you can use fonts (by order of complexity):
1) web pages (eg contributions) This one is simple, it's a matter of changing the css and possibly loading an extra bit of code (if you use google font for instance) It's easier to do it from the theme of the CMS around your civicrm
2) mails you can request to use a specific font (but you will need to inline the font, as most mail clients ignore stylesheets). If that font is already installed into the computer of the recipient, it will work, if not, you're out of luck
3) pdf you will need to register these fonts (as new files) and so probably have to code My experience with pdf is that it has several limitations (eg it isn't always unicode and you might lost some accentuated characters)
The last one I haven't tried, but can point you to were you might need to change the code: CRM/Utils/PDF/Utils.php CRM/Utils/PDF/Label.php
CiviCRM uses http://www.tcpdf.org as the library to convert to pdf, check there first
If you have full control on your server, you can as well configure civicrm to use wkhtmltopdf, it should be as simple as the case 1 to add custom fonts to your pdf