Fonts are an important, but often overlooked topic in any discussion concerning document conversion. Almost everyone can agree that the final result of a conversion is judged based on how well it replicates the original document and fonts play a significant role in this regard.
On most computers fonts are stored in a single location and have file extensions like .OTF or .TTF (Open Type Font and True Type Font). The specific font file chosen by the document author is referenced by editing applications to determine things like the space that individual characters take up and also to display a preview of the document. Note that the name of the font in the author's editing environment might be radically different than the name of the font file.
An important point to remember is that when converting a document from one format to another, the font files referenced in the input document need to be available to the application performing the conversion. This usually isn't an issue if the conversion is taking place on the same machine as the authoring.
Many font files are copyrighted so unless the font license explicitly allows multiple installs, it is a violation of the font owner's copyright to simply copy a font file from one computer to another. Most commercial font licenses allow for a limited number of installs, so the same purchased font can be used in the authoring, converting, and printing environments.
Sometimes a specific font chosen by the document author isn't available in the conversion environment. When this situation occurs, the application performing the conversion has to choose which available font to apply instead. As long as there are fonts of each type used in the initial document (sans-serif, serif, monospace, etc) available to the conversion software, then the final converted document will retain the original styling. However, font substitution may cause lines to break differently between the original and converted document and, under certain circumstances, may even cause overflows or other undesirable outcomes.
The best practice is to have the same fonts available in both the authoring and conversion environments.