"abstruse", "esoteric" and "recondite" are terms applied to "text that is not understandable to many people". Colloquially one may say "This book is heavy reading" or "This book is a heavy read".

In linguistics it would be called "high register." This would be the most accurate term. I think commonly, it would be called "technical," "advanced," "dense," or "a heavy read" depending on the context.

Thank you for your comment, Richard. I knew about the word 'incomprehensible'. But I didn't mean it. In fact, I meant a text that has lots of complex words. And most of uneducated people can't understand them. I can give you a clear example: The texts that are written in newspapers. Most of them have full of complex and strange words. That is what I mean. In Persian, we call it 'Heavy text'.

Normally, if the text cannot be understood by someone, then I would use the adjective 'incomprehensible', but the writer should make it clear who the people are who would not understand the passage, e.g.: ... The text is incomprehensible (not understandable) to those who are not engineers. ... Computer program coding is incomprehensible to me.