I need to get files with data to run my own program for each file or bunch of files. What is the best way to receive those files?
Do I need to set up my own server that expose API for file upload? Should I use S3 and set up a server that fetch files from it automatically? or Maybe DropBox? Or should I use SFTP server? The amount of files are not big (max 1Giga per day)
I'm using Spring boot for my own server
By using SFTP, you can listen to changes (e.g. file upload) in a remote directory by connecting a listener. SFTP, in my opinion, is a great choice for file integration.
You said you will be using Spring Boot. Spring has a project called Spring Integration, which has SFTP support.
Spring Boot has a starter called spring-boot-starter-integration useful for bringing the best suited Spring Integration versions to your Spring Boot project. Spring Integration SFTP is really easy to use, since a lot of abstraction is provided by the framework.