PA sent you this letter because they are questioning your claim for unreimbursed employee expenses. If you don't comply with the letter, then they will disallow your deduction for these expenses and recalculate your return without the deduction. This would mean you would either receive less refund than you were expecting or you may owe additional taxes.
Here are some links with further information -
https://revenue-pa.custhelp.com/app/answers/detail/a_id/3221/~/understanding-your-829-letter.
http://www.revenue.pa.gov/GeneralTaxInformation/Tax%20Types%20and%20Information/PIT/UnreimbursedExpe...
http://www.revenue.pa.gov/FormsandPublications/FormsforIndividuals/PIT/Documents/rev-775.pdf
http://www.revenue.pa.gov/FormsandPublications/FormsforIndividuals/PIT/Documents/rev-757.pdf
