Document analysis involves analyzing documents for specific purposes like requirements gathering, risk identification, test case development, forensic analysis, claims etc.
For example, any requirement for which we are unable to write a proper test case is an ambiguous requirement, hence a risk. With this as the intent, if one analyse the requirements document, many requirements related risks can be unearthed. So, is the case with design documents, for that matter all project related documents.
In forensic analysis, project documents are analysed to create substantiate claims made in projects.