Your breast milk is not low in fat. There is no such thing as "skim" breastmilk. Most likely your daughter is just a small child. If you want to convert the baby to formula for other reasons, that is your decision, but you shouldn't do it to try to make the baby gain weight.
One trick if you are pumping is that the foremilk (the first milk that comes out) tends to be lower in fat than the hindmilk (the later milk). So you can discard some of the first milk and feed her bottles of the milk that is the more concentrated hindmilk. That way even though she drinks the same amount she will get more calories and fat. You may have to work to increase your supply a bit to do this. I have also heard of people letting the milk settle and "skimming" it so to speak to remove some of the liquid and make it more concentrated.
Please consult a lactation consultant or La Leche League leader before you stop pumping (again, unless you just want to stop pumping for other reasons, perfectly fine, your decision). Your daughter is getting benefits from your breastmilk that do not come in formula.
By the way, I have two small children so this is an issue I have experienced often in the past. My 3 year old was 8 pounds, 11 ounces at birth and now at 3 and a half is only 24 pounds. This is pretty small but the reality is that I was a small child and my husband was a small child, so it only makes sense that she is also small. My son is the same way.
Good luck.
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Editing here- there seems to be some sort of misconception that a child in a lower percentile for weight is somehow unhealthy. Remember that the children in the upper percentiles are big for their age and those in the lower are small for their age. The only reason for concern with regards to that is if your baby suddenly falls from 60% to 5%, then there may be an issue.
Also, check the growth charts the doc is using. Many doctors in the US use growth charts made by formula companies. The numbers are different and reflect the weight gain pattern of formula fed babies. Your doc should be using a World Health Organization chart that is made for breastfed babies. The weight gain pattern of a breastfed baby is diffferent.
Deciding to breastfeed or not is a choice only you can make and only you can decide what is best, it's just that so many women are forced to make this decision without proper information.