Is taking documents from a competitor company and copying their contents with some changes, for the purpose of establishing a startup company in the same field, considered theft and forbidden? And does sin befall one who did not participate in this act, or is the company built upon error?

It is not permissible to infringe upon the documents of another company by copying them for personal benefit without permission, because Allah Almighty says: "And do not transgress. Indeed, Allah does not like transgressors" (Quran 2:190). Your partner's knowledge and consent to this infringement do not absolve the sin, but they do not make the partnership or its earnings unlawful. You must explain the impermissibility of this to your partner, unless you have been authorized to do so by someone who possesses the authority, or if the documents can be obtained through a permissible means.