Ok...finally success! Here's a manual for future users.
How to convert PDF files and transfer to KOBO in order to work as a full ebook:
1) either use a PDF editor to extract the text and copy to a Word processor (didn't worked most of the times for me), or go to this site
http://www.convertmypdftoword.com/
(best results for the tests i've done)
2) After having the text in the word processor, export it to PDF format
3) Use Calibre software to convert the PDF to EPUB format
4) Use Adobe Editions to transfer the EPUB file to your Kobo.
Done.
After a lot of research and the help of the community i realized that there are a lot of problems with the Ereaders handling PDF files. You can easily transfer a PDF to your ereader, but it transfers as a sort of image and doesn't let you use all the capacities that a normal ebook does. This is way you have to extract the text from the original PDF file. As Mrs_Often refered you have to "make a new PDF with text instead of the images of text"