91st PA: Pennsylvania Archives, Register of PA Volunteers

[source: Pennsylvania Historical and Museum Commission, Bureau of Archives and History, Record Group 19, Records of the Department of Military and Veterans' Affairs, Registers of Pennsylvania Volunteers, 1861-1865]
[Thanks to Ed McLaughlin for letting me know about these records!]

Field and staff officers [pages 1-2]: page 140
Company A [pages 3-12]: page 141; page 142; page 143; page 144; page 145
Company B [pages 13-22]
Company C [pages 23-32]
Company D [pages 33-44]
Company E [pages 45-54]
Company F [pages 55-64]
Company G [pages 65-74]
Company H [pages 75-84]
Company I [pages 85-94]
Company K [pages 95-102]
Company K, unassigned, and regimental information [page 103]
Engagements [page 104]

Registers of Pennsylvania volunteers