When a file that was shared with Everyone is put into a community and someone opens that file through the community, the URL he/she sees is relevant to the community. Meaning that when he or she then copies that link and forwards it to someone else that is not a member of the community, they will get an error saying they are not allowed to view the file even though the file is public and can be opened by them if they have a direct URL.
Please change so that the system, upon detecting the file will forward the user to the right URL or make it easier for the user to get the right link.
ps. I am aware that you can get the right link from going to the About tab and using the 'Get Links' option to get the right link. This is however not something most users know or will remember. That feature is way too hidden.