Thanks for your reply,
Yes I have a text file with 3000 PDB IDs. I want 3000 fasta files of the corresponding IDs downloaded.
Do you mean, I need to go to this website (which is not working) http://www.rcsb.org/pdb/files/fasta.txt?structureIdList={} and enter parallel -a pdb_list.txt curl -o {}.fasta?