Follow the instructions in the letter you received from PA Dept of Revenue and mail a copy of your W2.
Your PA tax return should have included a Schedule W2S that lists the W2 information. Either this form was missing or they have additional questions. Rather common for PA to ask for the W2 if the PA income (box 18) is significantly different from the medicare wages (box 5) on the W2. Sometimes happens if you worked or lived in another state than PA as the employer may have applied non-PA laws in determining taxable income. Or the amount of PA taxes withheld as shown on the W2 does not match PA's records.
