You will need to convert the array of bytes to a string with StringUtils.EncodeBase64. Push notification messages are limited to 4kb (if I remember correctly). Check the string length to see whether it fits.
You will then need to decode it to an array of bytes on the client with StringUtils.DecodeBase64.
I do not know what is better to send the file or just a info with download coordinates (firebase storage).
Normally the 4KB should be enough, but you never know exactly.
What do you mean?