PDB or mmCIF (.cif). mmCIF is converted to PDB locally so editing/coloring/export all work the same either way — files are never uploaded anywhere.
.pdb · .ent · .cif · .txt
Click a residue to select it, shift-click to select a range. Selecting in the sequence highlights it in 3D — and vice versa.
Exports are saved as standard .pdb files. CONECT records and header metadata are not preserved; coordinates, chains and B-factors are.