Oh, that seems to happen with some apks for some reason, I was also really confused at first. :P
You have to use an apk installer, the one I use is SAI, which you can find on the Google store.
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Thanks for your answer ! I figured out that my mobile Firefox was corrumpting the apk each time : even with a package installer it will not see it as an apk.
When I tried to download the apk from my computer, put it in my personal cloud and get it from the cloud to my phone, my package manager would see it and I could finally install it.
Hope it could help some people with the same issue in the future !