When clicking the "PDF File" link on https://projecteuclid.org/euclid.afm/1485893123 a HTML file gets downloaded instead. The download works fine with Qt 5.12 and with Chromium 76.

Looking at the requests made with mitmproxy, the following happens:

• There's a GET request for https://projecteuclid.org/download/pdf_1/euclid.afm/1485893123 with the correct "Referer: https://projecteuclid.org/euclid.afm/1485893123" header.
• The server answers with the PDF file. With Qt 5.12 (or Chromium), that PDF file gets downloaded/displayed.
• With Qt 5.13, there's a second request mode without any Referer header.
• The server replies with a redirect to https://projecteuclid.org/euclid.afm/1485893123 which is the HTML page getting downloaded.

With simplebrowser, this causes the PDF not to be displayed in PDFium. With QWebEngineSettings::PdfViewerEnabled set to false, the HTML (rather than the PDF) gets downloaded.