Power BI let's you download pbix from UI but you can also export it via API.

Download a report from the Power BI service to Power BI Desktop

Power BI Rest API: Reports - Export Report

Power BI team also released PowerShell module for PowerBI. So I explain how you can download pbix file via PowerShell.

Check the Report Id

Open a report on Power BI service and confirm the report id.

PowerShell

Open PowerShell as Administrator and install PowerBI module.

Install-Module -Name MicrosoftPowerBIMgmt

Then login to Power BI service.

Login-PowerBI

Finally download pbix file.

• Replace ReportId
• Replace OutFile

$date = (Get-Date).ToString("yyyyMMdd")
$ReportId = "960e6b3d-b1ce-49fb-8e87-201d6e02f1e0"
$OutFile = "C:\Users\kenakamu\Desktop\test$date.pbix"

Invoke-PowerBIRestMethod -Method GET `
-Url https://api.powerbi.com/v1.0/myorg/reports/${ReportId}/Export `
-ContentType "application/zip" -OutFile $OutFile

It fails if the file already exists, so I use date to make unique name.