I have an older version of iOS, how can I download a version of the app which is compatible?

The App Store allows developers to offer a compatible, earlier version of an app for earlier iOS version users based on Apple's support article here: http://support.apple.com/kb/ht5919

Download an older app version:

1. Open the App Store on your device running iOS 4.3.3 or later.
2. Go to the Purchased screen. For iPhone the Purchase screen is in the Updates tab.
3. Select the app you want to download.
4. If a compatible version of the app is available for your version of iOS simply confirm that you want to download it. For example, after purchasing iPhoto for your iOS 8 device, you can install an earlier compatible version of iPhoto from your iOS 7 device.

Limitations:

In order to have access to an older version of an app, both the older and newer versions need to be the same app, in a technical sense. This is not a distinction you normally need to be aware of, except in this specific circumstance.

In the case of many iOS apps, a change in major version number will signal a new app so that you will not be able to backdown from v.3 to v.2 like you might for v.2.5 to v.2.1. ( Not sure which version you are using?)

TextExpander touch 3: You cannot backdown from v.3 to v.2. You can backdown from v.2 to v.1.

PDFpen for iPhone, PDFpen for iPad: You can back down to any previous version.

PDFpen Scan+: You can back down from v1.5 and later to any previous version.

(Applies to iOS 4+.)

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