Check this option to subset fonts which are not completely used in the file. This means, if only the characters A and B are used, only A and B from the font will be embedded. This helps to reduce file size and is recommended. The percentage is the percentage of the font used in the file and the font still being allowed to be subsetted instead of the entire font being embedded. For example, if this is set to 50%, if less than 50% of the characters in the font are used, the characters used will be the only characters embedded. If more than 50% of the characters in the font are used, the entire font will be embedded.