I deleted the PDF Document Scanner app for iPhone, but subscription has been renewed anyway. Why?

Subscription and cancellations are managed by Apple iTunes, not us.
Please read the cancellation conditions of Apple here:

There you can read, that cancellation must be done 24 hours before the end of a free trial or subscription period
to avoid being charged.

This means, if you cancel the subscription less than 24 hours before your renewal date, your subscription might be
renewed by Apple for one more period.

To read more about managing subscriptions, please visit Apple: