This applies to a Windows client connecting to a Linux host, where fonts are missing or not properly displayed (e.g. replaced by squares). Usually the application is a legacy one requiring additional fonts to be installed on the system.

The NoMachine display agent is able to draw missing fonts from client side, if such fonts are installed on the server host. When fonts are not properly displayed inside the NoMachine session, check if they are effectively installed on the remote computer.

In some cases, however, it may be convenient for improving performances or solve rare cases of fonts not drawn properly, to provide such fonts on client side. To do that, install the NoMachine add-on fonts on the Windows computer. They can be downloaded from the download page of the client package for Windows. Be sure to install add-on fonts for 64bit if the client is for 64-bit or 32-bit add-on fonts if client is for 32bit. 

If the add-on fonts still don't help, it means that they don't inlcude the missing font. The display agent logs from server side report which fonts are missing, e.g.:
nxagentLoadQueryFont: Can't load font [-xos4-terminus-medium-r-normal--13-94-100-100-c-80-iso8859-1].

Message in older version of NoMachine is:
nxagentLoadQueryFont: Cannot realize font [-xos4-terminus-medium-r-normal--13-94-100-100-c-80-iso8859-1].

It's possible to add the missing font(s) manually as explained below.

Fonts are loaded by the client on Windows from the following paths.

On Windows 64bit:
%PROGRAMFILES%\NoMachine\share\fonts\<subfolder>

On Windows 32bit:
C:\Program Files (x86)\NoMachine\share\fonts\<subfolder>

The loader will parse every <subfolder> looking for fonts.

Copy the fonts subdirectory from the /usr/share/fonts directory from the remote Linux computer to the 'fonts' folder on Windows.

For example, on Windows 64bit, copy /usr/share/fonts/misc/ to %PROGRAMFILES%\NoMachine\share\fonts\