This issue happens when you try to download the file from a mobile device, 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.