There is an undocumented limitation of the PDF Report Viewer.

The PDF report viewer will not preview reports that are very large. When a report of several hundred pages (GL Dimension Statement without significant filters for example) is run the report will not be displayed in preview mode inside the app but instead will download the PDF. I'm sure this is by design but it should be added to the limitations section of the following documentation.

https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/analytics/preview-pdf-documents