Sometimes text selection not possible, not accurate, not usable. Why ? How to solve ?

Hello . . .

Sometimes I have to work on PDF files.

With the text in some files I have no problems: I can select it and modify it to my liking.
In other files instead the modification is impossible: the selection extends for most of the page, sometimes in the shape of a cut egg, sometimes in the shape of a rectangle.

Acrobat, once, when opening an image, it automatically started the conversion and after the conversion, the text that was previously part of an image, became immediately and totally editable. Now no longer. Why ?

If I manually start the text recognition from "Scan and OCR", nothing changes!

I found a solution: I export as an image, and - in the many ways offered by Acrobat - I open the image, or import it, or create it from a file, but the result is not flawless, it is not presentable, it looks like an acquisition from low resolution scanners, especially if compared to a successful acquisition.

What's the explanation? Is there a more professional solution?

Greetings.

J.K.