The Pro version is sold separately (as in-app purchase) for the Android, iOS & macOS versions of Diarium. On Windows, you get the Pro version by buying the app itself.
After installing the free app version on Android, iOS or macOS, there will be a 7-day trial period in which you can test all Pro features before they are being disabled unless you purchase the Pro version.

What features does the Pro version contain?

Sync across all your devices using your personal Cloud storage
Add an unlimited amount of attachments to your entries
Export your diary entries as Word file. That way you can also print your entries
Connect other apps (Twitter, Facebook, Instagram, …) and your activity will automatically show in your entries
Add weather information to your journal entries
Choose your preferred font and font size
One-time payment - NO subscription!

Is the Pro version a subscription?

No, the Pro version is a durable one-time purchase - no subscription!

Do I need to buy the Pro version for each device separately?

The Pro version license is bound to the respective App Store account (e.g. on Windows: your Microsoft account). That allows you to use the same license on multiple devices of the same platform (e.g. Windows). Because different platforms are using different App Stores (e.g. Windows: Microsoft Store, Android: Google Play Store, …), you need to buy the Pro version separately for each platform you want to use it on. iOS and macOS are treated as two different platforms in that regards - even if both iOS & macOS app are downloaded with the same Apple account.
Example: You want to use the Pro Version (e.g. the Cloud Sync) on five devices: two Android phones, one iPhone, one iPad and one MacBook. The Pro Version then needs to be purchased 3 times (once for Android, once for iOS, once for macOS).

Why do I need to buy the Pro version for each platform separately?
In order for one license to work for all platforms, I would have to implement my own licensing system, instead of using the licensing system provided by Microsoft, Google & Apple. This not only takes up time that I would rather use to improve the app and build new features, it would also cause additional costs which in turn could impact the price of the app - even for those who only want to use the app on one single device. Also, users would need to create a Diarium account that the licenses can be synced with, which is something I would want to avoid: Diarium should be as easy to use as possible, without having to worry about accounts, logins or signups. All these reasons brought me to the conclusion that a unified license (pay once, use on all platforms) would not have enough benefits to outweigh the drawbacks. I am always open to discuss alternatives, so if you think there's something to improve, please reach out to me.

How to restore the Pro version on a new device?

As described above, the Pro version needs to be purchased only once per platform. In case the Pro version does not automatically activate after switching the device and using the same App Store account, then re-initiating the purchasing process can trigger the download of the license (no worries - it is not possible to buy the Pro version twice with the same account). Additionally, in the iOS & macOS versions, there is a "Restore purchases" button on the "Pro version" page. If this doesn't work, it is also recommended to reset the App Store Cache in the operating system settings (Windows / Android).

[Android] I bought the Pro version before, why is it gone again?
If you previously bought the Pro version on Android but the app falls back to the free version, this might be due to the Play Store (perhaps mistakenly) reporting to Diarium that no Pro version license is present in the currently active Google account. Please make sure you are logged in to the Play Store app with the same Google account that you bought the app with previously. You can also try to restart the purchasing process - it is not possible to get charged for the Pro version twice on the same Google account. So if you are logged in with the correct account, this will result in Google Play recognizing the Pro version has already been bought and Diarium will enable it again.

If the above does not work and you are using multiple Google accounts on your device, you might have downloaded the app with the wrong Google account. You can try to reinstall the app (make a diary database backup in the app settings before doing so!) while making sure the correct Google account is set as primary account (button on the top right in the Play Store app).