I need to write into the text box on PDF but I don’t want to show the black text-box boundaries when I print that paper.
You can see this photo to help
Not sure about the origin of the PDF - but if it is a form you got from someone else, please note that there are more advanced tools to fill PDF forms than LibreOffice Draw.
Hello,
if it is a text box / shape: Right click the text box -> Line -> Tab: Line -> Style: -none-
Hope that helps.