Hey and welcome to the Cryptomator Community 
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I’ll answer this question in a more general way. As you said, Cryptomator downloads the file but into the app internal storage. This storage is only accessible by this app (if the device is not rooted or someone is able to exploit the sandbox model of Android). Now we have to options: There are some built in viewer and editors included, like for pictures or text files but all other file types will the user be asked to open with an external app like a PDF viewer.
We delete those downloaded and decrypted files in internal storage when all vaults got locked. But when you shared a file with another app this app could create a copy of it that we don’t know about and can’t delete. So in general, just share files with apps that you trust.
You can archive the same as our auto wiper by clicking on System Settings -> Apps & notifications -> Cryptomator -> Storage & cache -> CLEAR CACHE but as I said we delete those files already so it is not necessary to do it, but I just wanted to mention it.
Edit: If you want some sources about the mentioned stuff, please let me know.