I'm dropping Create workflows that call external endpoints or other workflows - Azure Logic Apps | Microsoft Learn as the guide for using multipart/form-data in a logic app. If the PDF is getting corrupt, I would double check the HTTP connector settings. See if a different PDF file will work. If it does, then it could be an encoding error on the original PDF you're attempting.
As for alternatives, it depends. You can push the PDF file to some form of cloud storage and use an appropriate connector to pull it, e.g. Azure Blob storage account, FTP, or OneDrive/SharePoint.