Cutting and pasting a bit from my webpage to save time....
"Ectropian is when the eyelids are droopy and roll outwards. The droopy eyelid may collect debris such as dust, pollen and plant material from the environment. This may cause irritation to the eye which leads to discharge and a red eye. Dogs that have ectropion must be watched carefully by their owners for possible foreign bodies in their eyes, and the dogs' eyes must be cleaned and often medicated on a regular basis.
The opposite of ectropian is entropion which is when the eyelids roll inwards. If the eyelid is rolled inward sufficiently so that the hairs of the eyelid rub on the eye, much damage may be done to the eye. Dogs with entropion usually squint and have watery eyes. If the entropion is not corrected and the rubbing continues, ulcers often develop on the cornea and the cornea becomes pigmented. Vision may be lost. Dogs that have had surgery to correct entropion can not be shown.
Although entropion and ectropion are hereditary disorders, their mode of inheritance is complex. No single gene controls the development of eyelid conformation. Instead, it is a combination of genes that control eyelid size and shape, depth of the eye socket, size and shape of the eyes, head conformation and amount of facial skin. All of these genes work in concert to determine the relationship of the eyelids to the eye. Therefore if an eyelid conformation defect is to be eliminated, only those dogs without entropion or ectropion should be bred. This is particularly important if breeders are to maintain the correct close-fitting eyelids required by the breed standard for the Pyrenean Mountain Dog. Not too loose, but also not too tight. "
Now your question on whether it is 'common', entropion tends to be more common than ectropion in Pyrs. Not to say that it won't occur from time to time (particualrly in dogs with 'looser' heads. Best bet is to get a referral to an eye specialist if you are concerned. They will be able to give you correct information on how much of an issue it will be for your dog and whether there is any particular action required by you. In minor cases you may need to just take care as discussed above. If the case is severe there are other Things that can be done including operating to give the dog and 'eye lift' so to speak.
Espinay Pyrenean Mountain Dogs (Australia) - putting the Breed before breeding