There is only one file type that uses the .UD extension:
OmniPage User Dictionary
User dictionary created for OmniPage, an optical character recognition (OCR) program; contains custom words, such as proper nouns, technical terms, and abbreviations that have been added by the user; supplements the program's built-in dictionary, which assists in the text recognition process.
NOTE: OmniPage includes multiple dictionaries for different languages. Each of these dictionaries may have its own user dictionary. Therefore, several user dictionaries may exist at one time.
Updated: June 9, 2010