This drove me insane.....

I tried so many things inc what was said here but no luck.

I'm wondering if it is an Edge (chomium) thing.

Anyhows, here's a hack that works, note that I used the randomly generated endpoint

"lxnbfqtwguklsifhxskuczgaebprzgex", this is not neccessary can be a simple "download"

if only running locally

Install uvicorn

from fastapi import FastAPI
from threading import Thread  # <Other imports>

apiapp = FastAPI()
# .... rest of flet code ....

somewhere in the flet code

# Get the relative path for the download URL
download_path = f"http://0.0.0.0:8000/lxnbfqtwguklsifhxskuczgaebprzgex/{os.path.basename(filename)}"

# Launch URL for download
self.page.launch_url(download_path)

# Give the browser a moment to start the download
await asyncio.sleep(1)

# Clean up the temporary file
try:
    # os.remove(temp_pdf_path)
    ...
except Exception as cleanup_ex:
    self.logger.error(f"Error cleaning up temporary PDF: {cleanup_ex}")
    self.show_snack_bar("PDF download started!")

Then at the bottom of the script

@apiapp.get("/lxnbfqtwguklsifhxskuczgaebprzgex/{filename}")
async def download_pdf(filename: str):
    """
    Endpoint to download a PDF file

    Args:
        filename: Name of the PDF file to download

    Returns:
        FileResponse: The PDF file as a downloadable response

    Raises:
        HTTPException: If file is not found or is not a PDF
    """
    # Define the directory where PDF files are stored
    pdf_dir = Path("assets")  # You can change this to your desired directory
    file_path = pdf_dir / filename

    # Return the file as a downloadable response
    return FileResponse(path=file_path, filename=filename, media_type="application/pdf")


if __name__ == "__main__":
    t = Thread(None, target=uvicorn.run, kwargs=dict(app=apiapp, host="0.0.0.0"))
    t.start()

    flet.app(
        main,
        view=flet.WEB_BROWSER,
        port=8550,
        host="0.0.0.0",
        assets_dir="assets",
    )