This is from the Dropbox website. Which sucks. I have verizon and dropbox did NOT come preinstalled in my s4. So I don't qualify.:'(
Hope this helps
Is my Samsung phone or tablet eligible for the free space promotion?
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If you have a brand new Samsung phone or tablet with Dropbox already installed, you may be eligible to earn 48 GB of free space for up to two years. To qualify:
The Dropbox app must be pre-loaded on your device.
You need to complete at least five steps from our Get Started guide.
Your carrier must be participating in the promotion.
Make sure Dropbox is pre-loaded
Dropbox must be "pre-loaded" on your brand new phone or tablet, which means that the Dropbox app already came installed without the need to download it from the Google Play store.
If the Dropbox app is pre-loaded on your device, you may qualify for 48 GB of bonus space for up to one year.
Additionally, some devices that have the Dropbox app pre-loaded may also have the Dropbox "out-of-box experience," which means that the first time you turn on the device, you'll be asked to sign in to Dropbox or create a new account. If your device has the out-of-box experience, you may qualify for 48 GB of bonus space for up to two years.
The first Dropbox account that signs in to the app on the device will get 48 GB of free space (assuming the other conditions are met). Each account is only able to claim this promotion once.
Complete the Get Started guide
To claim your extra space, you need to complete at least five steps from our Get Started guide. Please note that some steps require your desktop computer.
Make sure your carrier is participating
Some carriers have opted out of the promotion on phones otherwise eligible. Among those not currently participating are the following carriers:
United States: AT&T
Japan: NTT DoCoMo
Verizon customers: Some Samsung devices on Verizon may be eligible for 48 GB of bonus space for one or two years. As described above, your device needs to be pre-loaded with the Dropbox app, or pre-loaded with the Dropbox out-of-box experience, to receive the applicable promotion. If your device does not have the Dropbox app pre-loaded, then it is not eligible.