Hi,
Welcome to share here!
Based on your requirement, we must grant users different permissions to the shared folder and the files within the folder.
1, For the share permission, we can give everyone change permission on the shared folder as following:
2, For the NTFS permission, we can grant users read permission on the folder:
3, Assign modify permission to users on the files with the folder:
Then you can modify the files and save the changes but can't use the “save as” option within this folder and can't create any files or folders.
Best Regards,