The following is from Autodesk.....
Issue
The following error occurs when opening or editing a drawing:
This object contains too many unknown fonts, and cannot be accurately represented within the editor. If you wish to preserve all font information, click OK below, and then click the Cancel button to dismiss the editor.
Solution
This message displays when the drawing being modified contains SHX fonts that were not part of the software installation.
Regardless of what caused the message, this message is benign and can be ignored by choosing OK, unless you need to modify text created with the added SHX fonts using the Multiline Text Editor. Consider the following scenarios.
The drawing contains SHX fonts, for which there are no TrueType font equivalents.
The Multiline Text Editor can only display TrueType fonts (TTF). To display text created with SHX fonts, the TrueType font equivalent of all default SHX fonts are provided in the software, so that text created with SHX fonts can display in the Multiline Text Editor. Also provided, are nine proxy fonts for SHX fonts that do not have a TrueType font equivalent. When a drawing containing more SHX fonts than there are proxy fonts is opened or modified, the error occurs.
The mtextmap.ini file maps the SHX fonts to TrueType fonts. If the file is found in the Autodesk software installation and the message displays, you need to add entries to the mtextmap.ini file to map the added SHX fonts to TrueType fonts.
Tip: Copy one font mapping that already exists in the file and replace the names of the fonts with the names of the fonts you are mapping. For example, copy txt.shx=Txt,0,0,0,2 and change it to MyFont.shx=Txt,0,0,0,2.
The mtextmap.ini file cannot be found.
Ensure the file exists and is located in the support file search path. If the file does not exist, copy mtextmap.ini from the Autodesk software installation CD to a folder listed in the search path.
--------------------------------------------------------------------------------