PBI Explorer is a free tool to

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  Take full ownership of your Power BI reports

  PBI Explorer gives you a code-first view of your PBIX/PBIP reports. All visual properties, settings, and relationships are fully exposed in Tabular Editor style.
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  Inspect differences between report versions

  PBI Explorer provides a git-like diff view between two reports, highlighting what's been modified or removed.
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  Understand how the data model is used by reports

  PBI Explorer provides a view of all elements from the data model used in the report.
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  View difficult-to-see report features

  PBI Explorer exposes all report features that are difficult to inspect visually (like bookmarks, filters, etc).

PBI Explorer is free and will always be free.