Essentially, this document says that there is not to be diffused the idea that one can know the name of one's guardian angel that that one is not to invoke the (unknowable) names of angels in prayer.
I have not found online an English version of this yet and I do not have the time to translate it at the moment.
What the CDF is warning about is the great spiritual danger that can come to a person from trying to invoke angels by name. The naming of angels is something that should be avoided. Not all angels are the friends of God and man. They are mighty beings that transcend our human nature. Some of them are bent on our eternal damnation. They can apply an angelic intellect in tricking us into the loss of heaven. They do not sleep. These are demonic enemies of the soul. Demons do not relent, unless God and His angels, unless the Church God's priest intervene. The naming of fallen angels is part of the rite of exorcism. It is perilous indeed to anyone without the protection and power that comes from the sacrament of Holy Orders to name angels.
This is a foolish and sentimental practice for most people who are into this sort of thing. But it is not benign. While it is not dangerous in all situations, it is dangerous in some. We just cannot know whom or what we are invoking outside the names of the angels friendly to God whose names appear in Holy Writ: Michael, Gabriel, Raphael.
Thank goodness for Father Z! I had heard on another forum that it was a good idea/practice to name one's guardian angel and did try it. I never really felt comfortable with it though...