PDBeMotif is a good way to search for sites, motifs and ligand interactions with a given PDB file.

pdbemotif
Use the ‘Upload PDB file’ to search the protein data bank for structures.

pdbemotif How to Search for Structural Features in the PDB
You can then upload your PDB into the server for searching.

Note: the structure that you are searching with does not need to be in protein data bank.

In addition, you can use SPASM structural motif hunter– either download or run it from a server. SPASM uses a max of 500 atoms so you will probably need to edit your PDB file for the specific motif that you are searching with. If you want more information about SPASM then click here.