Tor Browser: How to set PDF handler to 'Always ask' / 'Save File'?

Qubes-Whonix
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I am using Tor Browser in the default anon-whonix disposable.

For security I want to set the PDF application handler to something else than “Open in Tor Browser”.
Like “Always ask” or “Save File”.

Disabling internal PDF with pdfjs.disabled is not an option, as increases the fingerprint wiht JS enabled.

Tried

pdfjs.previousHandler.alwaysAskBeforeHandling = true
pdfjs.previousHandler.preferredAction = 4

without desired effect. How might I make PDF handling safe then?

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