Could you please share a bit more about your scenario?
How do you display the PDF file content within your app? Using PDF Viewer Control?
For your first question, do you want to send file stream to REST API to create a PDF and receive the file back in PowerApps? Do you want to receive the PDF File content or PDF File URL back in your app?
Please check and see if the following video would help in your cenario:
If you want to send file stream to REST API to create a PDF and receive the file back in PowerApps, I think the combination of Microsoft Flow and PowerApps could achieve your needs.
On your side, you could consider trigger a flow from your app, then within your app, send file stream to REST API using "HTTP" action in Microsoft Flow to create a PDF, then add a "Respond to PowerApps" action or "Request - Response" action to save the generated PDF file content back to your app.
More details about starting a flow from an app, please check the following article: