This article provides detailed explanations of the Comment Reports available in PPO:
Comment Detail Report
The report includes all the fields on the Comments entity for all comment records and projects. If the “Include Project Information” checkbox is selected, the report will also include all the fields on the project entity.

Comment History Report
The Comment History report provides a complete overview of how the various comments have changed over the lifespan of the project. The report also has various report parameters that can be applied.