When you upload a file from your phone to Dropbox, it isn’t automatically visible to other people. It just sits there in your private Dropbox folder. For other people to be able to download it to their phones, they first have to copy it to their Dropbox folder. This cannot and does not happen automatically.
So how do they copy your files to their Dropbox folder?
First of all, once the file is in your Dropbox, you can access it from your PC/Mac/Phone/Tablet/Browser and do whatever you like with it. There are many ways you can send a file to someone else on the internet, for example you can post it on the forums.
Dropbox itself offers 2 ways to share files:
1. By creating a public folder. Each file that you put in a public folder (again, you have to copy them there first) gets a public link through which anyone can download it. You can post these links on the forums, send them via email etc.
2. By creating a shared folder. All files in your shared folder are visible to other people you invited. You have to manually invite specific people though, so this may be less convenient.