Under Polish law, a written signature is not necessarily required for a valid contract - contracts are generally valid if legally competent parties reach an agreement, whether they agree verbally, electronically or in a physical paper document (Art. 60 and Art. 66 of the Polish Civil Code), unless the law requires a specific form to conclude a given contract.
Depending on the type of contract, Polish law provides for varying minimum-level formalities. Sanctions for not keeping the required formality vary—they can either cause the agreement to be invalid or make certain effects of a contract ineffective.
If Polish law does not require a written form to be used, the "document form" is sufficient (e.g., a pdf copy, email, etc.). Wherever Polish law specifically requires a written form (with ink signature), a qualified electronic signature may be used instead. This is subject to the other party consenting to the electronic form being used, and sometimes technical limitations. However, qualified electronic signatures will not work if the law requires a stronger form (e.g., writing with notarized signature, or notarial deed).