The reason you're reading this is probably because something weird or maybe even annoying happened to you when trying to open a MindNode file. There was an error message about fonts missing. This error occurs when a document uses fonts that are not available on a certain device, operating system, or operating system version.
Either, fonts were installed manually on a device (e.g. someone installs a font on the Mac that they found on the web, sets it for a node, saves it, and then opens it under iOS) or through different operating system versions (e.g. Apple adds new fonts, you set them for a node, save the document, and then open it on a device that does not yet know this font). This then leads to the error message in MindNode.