If it's stored as ASCII/UTF8 text, then it will be compressible because it's just numbers. On the other hand, if they somehow have a number format that spans that many digits then it'll already be perfectly efficient.
Um, sure. But I don't think anybody really cares if pi is compressible because it's ASCII representation of numbers. At best, you'd be getting some constant factor improvement due to the unused bits, but there's nothing specific to pi in that.