You can check specification of [pdbx_seq_one_letter_code][3] and [pdbx_seq_one_letter_code_can][4]. These items are generated from the depositor-provided sequence on the PDB side. I suppose that the software used is [MAXIT][5].

Note: these are not [PDB files][1]. They are distributed by the PDB organization, but the file format is mmCIF (or, more formally, [PDBx/mmCIF][2]).

[1]: https://en.wikipedia.org/wiki/Protein_Data_Bank_(file_format)
[2]: https://mmcif.wwpdb.org/
[3]: https://mmcif.wwpdb.org/dictionaries/mmcif_pdbx_v50.dic/Items/_entity_poly.pdbx_seq_one_letter_code.html
[4]: https://mmcif.wwpdb.org/dictionaries/mmcif_pdbx_v50.dic/Items/_entity_poly.pdbx_seq_one_letter_code_can.html
[5]: https://sw-tools.rcsb.org/apps/MAXIT/index.html