This issue happens when you try to download the file from a mobile device that has not the Google Slides app installed. So the link opens in Google Drive app and downloads as a PDF file.
To get the .pptx file you have to:
- Install the Google Slides app. Get it here for Android or iOS.
- Click the “Download as PowerPoint” button and the presentation will open in Google Slides app.
- In the app menu select the option “Share and export” ⇾ “Save as” and a dialog will appear to select the format.