Hi and welcome to Kirby.
You might want to check out this post, but I don’t know if that has any influence on the files inside the folder.
After reading up on umask it seems that this setting defines the read/write permissions of folders and files: https://askubuntu.com/questions/44542/what-is-umask-and-how-does-it-work
In general, folder permissions should be 755, and file permissions 644.
Edit: An option would be to change the file permission settings via a hook whenever a page is created via the Panel.