Can I trust a PA for restylane injections?

I suggest going to an expert for fillers and ensuring you get the most comprehensive workup for your face.  Best, Dr. Emer.

Physician Assistants, PAs, are what are known as "mid-level providers". They, as well as RNs, are qualified to perform cosmetic injectables. Many plastic surgery practices employ PAs or RNs to do injectables, thus freeing up the surgeon to do surgeries. On the other hand, many surgeons do their own injectables. Ultimately, the training and experience of the individual are what count. Ask your PA how many injections he or she has performed, and how good are they at correcting any problems that arise. I wish you all the best!

It may surprise a lot of people to know, but almost every type of provider (MD/DO/PA/NP), regardless of specialty, gets most of their training on Botox and dermal fillers AFTER all their formal training. For example, as plastic surgeons, we spend a lot of time learning and practicing the intricacies of delicate surgery; for dermatologists, they are learning to identify the many subtle differences of what may look like just a brown spot to a lay person. While we do all get at least some training on Botox and fillers, I have known many senior residents that weren't completely confident doing these procedures on their own upon graduation. And while many here strongly advocate for board-certified plastic surgeons or dermatologists performing these procedures, I can tell you, as a board-certified plastic surgeon, that I wasn't ever scrutinized or asked about my botox techniques as much as I was my surgical skill set. 
So, the answer is that you want to go to a highly trained, experienced provider of any type that has good reviews and has performed many procedures. 
I have worked with a PA who, by himself and only doing injectables, achieved Diamond status (top 3% in the country) with Allergan, the manufacturer of Botox and Juvederm. That meant he injects more, by himself, than 97% of any practice that offers those services in the entire country. And he has 4.9+/5 stars on every review site he can be found on. 
So I can assure you, when given the choice to see him or me, for example, you most certainly should prefer me doing any of your surgical procedures, but you ought to very well prefer him to do your botox or fillers. 
Lastly, an artistic eye matters. While any contributor here may be able to learn the techniques - how to inject a filler, for example - not everyone will necessarily have an eye for it, nor may the provider's style always suit your desires. Ask to see before and after pictures and you'll quickly see if this is a provider you can trust.
Hope that helps!