Open Babel should do the job (http://openbabel.org/wiki/Main_Page). It can convert molecular structures form one format to another (including sdf).

PyMol is also able to convert a pdb to sdf format.

"Easy Convert" http://biochemlabsolutions.com/Molecule%20Docking/FORMATS/Formats%20PDB%20PDBQT%20SDF%20MOL.html

https://cactus.nci.nih.gov/translate/

You can convert your pdb file into add file using PyMol.

The above-mentioned solutions should work. I am just adding another one.

You can convert file format by BIOVIA Discovery Studio also. It worked great for me.