There are rules for which characters can appear in XML documents. For example, & must be represented as &. This is not an IS/TN issue. It’s a contraint imposed by XML. Please research the web for this info. Here is a wiki article that may be helpful for you.
If you’re using pub.xml:documentToXMLString, set the encode input variable to true (you should always do this) and it will convert the restricted characters to their appropriate character entities.
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