The only evidence I have that a Samuel McDonald served in the 91st Pennsylvania is that the pension index by regiment (1) includes him in the 91st. The regiment listed on the card is '91'. However, since neither the index to the compiled military service records, nor Bates, nor the Pennsylvania Archives Civil War Veterans' Card Files have a Samuel McDonald in the 91st, I suspect this is an error. The numerical pension index confirms that this is really the Samuel McDonald who served in the 97th Pennsylvania Infantry (from 3 March to 28 August 1865), according to the Pennsylvania Archives cards.
According to the pension index (1), he served in company I [?] of the 91st Pennsylvania Infantry, and also in "E 6 US Ca[valry?]" (1).
On 11 August 1866, his father applied unsuccessfully for a pension (1).
1 pension index, by regiment (Samuel McDonald)
2 extract from the numerical index to pensions, 1860-1934, NARA record group 15 (Samuel McDonald)