adding fonts ubuntu

how do I add additional fonts

How is this question associated to LibreOffice?

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Fonts are managed by the operating system - so turn to your operating systems documentation on how to install new fonts. Just be aware that PS Type 1 fonts are not supported with LibreOffice (starting with version 5.3)

Hint: For me on Ubuntu 19.10 double click on the .otf or .ttf file is sufficient to start application font-viewer and clicking Install button to install the font(s)