pdf to epup one line to paragraf

I have a lot of books in PDF format and want to convert them to epub format. Very often the result is not good. One line in PDF is converted to paragraph at epub with one empty line between paragraphs. I tryed a few things without success. I found this recommendation "you're getting a line break after every line (basically, every line gets turned into its own paragraph) - it's just that at your preferred font size the new "paragraphs" don't fit horizontally on a single line.

This is very common when converting from PDF sources.

To fix this the right way, do this:

Open the conversion dialog as usual
Select the desired output format as usual
On the left, click on the "Heuristic Processing" category
Check "Enable heuristic processing" to enable the rest of the controls on the page
Then (and this is what will actually fix this) check "Unwrap lines". Hold your mouse over the checkbox to read the help text in a popup. Please note that this isn't 100% accurate due to the nature of PDFs, there will still be lines where the problem manifests itself, but much, much fewer. Optionally adjust the line unwrap factor right underneath (explanation follows).
Check or uncheck any other options on that page - they control different things, not paragraph unwrapping.
Convert and check the results. Retry with different unwrapping factors is not satisfactory."

This did not work in my case.
I will appreciate any help