Page Load takes too long to load

Question

Hi i am currently developing a page that generates a pdf file. my problem is when i call the page that generates the file it should have a default text and image that says "generating file... please wait..."
but the problem is the controls and even the texts are not displaying until the function that generates the pdf is done... how can i make my page display all the texts and images before executing the function? and i also need the function that generates the pdf file triggered upon entering the said page.

help please....