pnmenlarge reads a PNM image as input, replicates its pixels N times, and produces a PNM image as output.
pnmenlarge can enlarge only by integer factors. The slower but more general pamscale can enlarge or reduce by arbitrary factors, and pbmreduce can reduce by integer factors, but only for PBM images.
If you enlarge by a factor of 3 or more, you should probably add a pnmsmooth step; otherwise, you can see the original pixels in the resulting image.