Supporting documents accompany Form I-140G through the official portal, following the instructions USCIS issues after the online application and fee. An employment offer letter belongs to the separate conversation about what your spouse may lawfully do, not to a guess about what the portal wants. Send what the instructions call for, and keep the rest organised.
Keep two files, not one
One file holds the material the official instructions request for the immigration case. The second holds the employment record, the offer, the professional licence, the credential evaluation, that counsel may need when advising on the spouse's ability to work. Mixing them tends to produce either an overloaded submission or a missing document at the moment it matters.
Label each item with its purpose and the date it was obtained, since some records expire before a case concludes. Give each file a short index and keep it current, since the two will be consulted by different people at different times. The immigration file is read against a set of instructions and should contain what those instructions ask for, no more.
The employment file is read by counsel advising on what the spouse may do, and by the eventual employer, and its contents have their own expiry dates: police certificates, medical examinations, credential evaluations and licences all age. Recording the date each item was obtained prevents the common waste of discovering that a carefully assembled document is now too old to use.