Is this the default ? If not, how I can do it ?
Type in Illy is vector artwork which may be described as text with a specific appearance, not just text; you may change it to shapes by outlining.
And the artwork you have created is all there is: there is no background, unless you create an object serving as a background.
You may say that it corresponds to a transparent background in other applications, at least in appearance.
If I have a PDF file and I want to write transparent text on it , I must:
open it in AI and after add text(since is default) and save as .pdf again ...???
that is correct. You open it in AI and you have an option to use area text, you first take the text tool and drag to make a text frame, which wors like it does in a layout program. Or you click the text tool any where on the canvas and just start typing if you want to make another line you either hit the return (enter) key or for soft return use shift/return (enter).
The default is a transparent background.
All art i Illustrator is on a transparent background unless you add an object behind other objects that has a fill color.