How to install reader on iOS 5 I have iPad 2 with iOS 5.1
I am developer and I need it to test apps.
But 99% of time nobody use it.
I decided to install ebook reader to reads fb2, mobi and other ebook formats.
But all readers I found do not install on iOS 5.
iOS 6 is minimal requirement.

Is there way to install older version of app?
Or upgrade just to iOS 6 (I know impossible)
Jailbreak? (I think it is impossible with version 5.1)

I am looking for solution to get some reader to work on iOS 5.1
Here's some information to keep in mind for the future. If you had ever installed a version of some app, then the App Store will allow you to continue either installing that app (if you had deleted it), or to install the last version compatible with that version of iOS. But if you never installed the app in question, Apple, the App Store will prevent you from installing it, even though there is obviously a version from App Store that will install if they'd let you. This is Apple's restriction and not something the individual app developers can decide.

Jailbreaking provides many options, but in my experience, introduces some new problems. I am just astounded by what, in particular, the Dev Team has accomplished, and more so what Jay Freeman (aka saurik) nearly single-handedly has accomplished. Cydia existed before the App Store, and is an implementation of Debian linux Advanced Packaging Tool. Developers began selling software there a good 5 months before App Store existed, as far as the public was concerned.

The new problems introduced by jailbreaking are merely that you really need to keep on top of things, and things change pretty fast in the jailbreaking community, which you necessarily need to become a participating member of if you want your device to continue working, or ever need help from someone because something stopped working. By and large, I found everyone in the jailbreaking community to be refreshingly shocking, but ultimately gave me a really great experience with it. But once my curiosity was satisfied, I found that maintaining my jailbroken device required more effort and time than I had to spend on it.

That being said, I just purchased a new iOS device, on the way... and because I really can't stand iOS7, I intend to get reaquainted with the jailbreaking community to mitigate what I just can't stand about it.

I recommend that you seek out more information from the dev team blog linked above, and also rediscover IRC, and the various jailbreaking IRC servers and channels, on Freenode and elsewhere (google helps), and involve yourself in that community in whatever positive way you can. And be patient with your desires. 

Sorry for the magna carte here! And best of luck on your adventure!