Download .pbix file with external user

Good afternoon.

I need to comment something later to ask the question.
A report was created from Power BI Desktop, which was published to the web with an administrator user, making the corresponding configurations so that from Power BI Services, external users to the company in the Applications part can observe such report. Therefore, the external user accesses power bi services (with his user), within "Applications" is the corresponding app, which "clicks" and can observe but not modify. Then you can make a copy of it being stored in your home. At that time, the saved app is observed in the reports section, which can be modified by the external user. Suppose that the external user does not want to modify the app from the work area, but instead wants to download the .PBIX file, to make the corresponding modifications from power bi desktop. It is here that the .PBIX file download option is gray, NOT LETTING DOWNLOAD THE .PBIX.
WHAT SOLUTION CAN YOU GIVE TO THIS TOPIC? I READ MANY POWER BI FORUMS BUT I CAN'T FIND THE SOLUTION.

Give