I need help about attaching a pdf file. After I'd erase the pdf file i've attached on the drawing. It doesnt show. Need help or advice on what to do about this

If you erased the PDF that you attached why would you expect it to still be visible?  Makes no sense.Attach a PDF using the PDFATTACH command.

Please how can I convert a jpeg drawing in autocadd.

You cannot "convert" a jpeg to a drawing using AutoCAD.  What you can do is attach it as an image and trace over it.  That's it. 

Hello Olayiwola, by using www.anydwg.com free sofware to convert PDF to DWG you can also try with an image file such as .jpeg but the result can be crude.
Don't rely on it for a precise drawing, just for general views and you will have to treat the file a lot.
 
Find here an example of jpg to dwg
uploads/175428/600TT2-01.dwg
 philippe JOSEPH2014-12-05 08:17:42

Image files that undergo "conversion" usually are of poor quality and many people who take this "shortcut" spend more time editing the results than actually getting anything of value done.  The method I mentioned previously will yield better results.