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Let me know if you find a solution please. I gave up :/

It's not reliant to the HTML code at all. It all has to do with your Edge policy and default PDF file association.

Actually some times it is. I just confirmed with multiple US court agencies that we deal with that in the example I am talking about there is code that you can use to force the PDF app to load into the browser. Certain jurisdictions have laws that state that the info they provide (for example pdfs on their site) must open in a browser to be viewable with out the requirement of a 3rd party software