Hello there! Since your cat seems to like the Purina food and it does not upset her stomach, why not try Purina's Senior Feline formula? Feeding her kitten food will not sit well in her stomach, as kitten food is highly enriched and very difficult for an elderly digestive tract to digest. The senior food is formulated specifically for older pets and may be beneficial to Pearl.
I know you had mentioned that Pearl has always been a petite girl, but if she has lost a noticable amount of weight recently, I would recommend getting her tested for Hyperthyroidism, as this is very common in older cats, the key symptom being weightloss. This ailment is maintained by a simple pill daily and indefinitely. A geriatric blood profile would also be helpful to make sure that everything is still working okay!
To put weight on her, you can try canned food and see how she does with that. Any food change should be done gradually over a course of 5-7 days to avoid a stomach upset.
I hope this is helpful.