Not able to generate PDF via PowerBI API call in java

Can you try it in Powershell?  The below works for me:

# Calls the Active Directory Authentication Library (ADAL) to authenticate against AAD
function GetAuthToken
{
    if(-not (Get-Module AzureRm.Profile)) {
        Import-Module AzureRm.Profile
    }
    $clientId = "<your client id>"
    $redirectUri = "urn:ietf:wg:oauth:2.0:oob"
    $resourceAppIdURI = "https://analysis.windows.net/powerbi/api"
    $authority = "https://login.microsoftonline.com/common/oauth2/authorize";
    $authContext = New-Object "Microsoft.IdentityModel.Clients.ActiveDirectory.AuthenticationContext" -ArgumentList $authority
    $authResult = $authContext.AcquireToken($resourceAppIdURI, $clientId, $redirectUri, "Auto")
    return $authResult
}

# Get the auth token from AAD
$token = GetAuthToken

# Building Rest API header with authorization token
$authHeader = @{
    'Content-Type'='application/json'
    'Authorization'=$token.CreateAuthorizationHeader()
}

#Report details
$groupsPath = "<workspace guid>"
$reportID   = "<report guid>"
# 0. get page names - optional
$uri = “https://api.powerbi.com/v1.0/myorg/groups/$groupsPath/reports/$reportID/pages"
$Pages = Invoke-RestMethod -Uri $uri –Headers $authHeader –Method GET
# 1. export request URI
$uri = “https://api.powerbi.com/v1.0/myorg/groups/$groupsPath/reports/$reportID/ExportTo”
$body = "{ format : `"PDF`” }" 
# how to define the pages? powerBIReportConfiguration:{pages:[{pageName:"<page 1 guid>"},{pageName:"page2 guid>"}]}
# issue the export request
$FileExport = Invoke-RestMethod -Uri $uri –Headers $authHeader –Method POST -body $body
#save the ID - we need it later
$exportId = $FileExport.id
# 2. export request status URI
$uri = "https://api.powerbi.com/v1.0/myorg/groups/$groupsPath/reports/$reportID/exports/$exportId"
$percentComplete = 0
# repeat rendering status check until status is Succeeded
while ($percentComplete -lt 100) {
    Start-Sleep -Seconds 30
    $exportStatus = Invoke-RestMethod -Uri $uri –Headers $authHeader –Method GET
    $percentComplete = $exportStatus.percentComplete
    Write-Host $percentComplete
}
# 3. retrieve the rendered file
$uri = "https://api.powerbi.com/v1.0/myorg/groups/$groupsPath/reports/$reportID/exports/$exportId/file"
Invoke-RestMethod -Uri $uri –Headers $authHeader –Method GET -OutFile "test.pdf"