Reducing the size of PDF could introduce issues that were not present in the file.
The type may have been deemed acceptable for print at its original size, but after reduction to a smaller type size, it may not be.
Lines that were originally thick enough at the original file size are now thinner due to reduction.
Bleeds may not extend far enough.
Trim marks may be too close to trim or even within the trim.
Crossover images along the gutter may not align.
Enlarging the size of PDF could introduce issues that were not present in the file.
Raster images (lineart and halftones) may become low resolution. Scaling an image up in size effectively lowers its resolution.
Text or images could now be within the 1/4' safety margin and too close to the trim.
Crossover images along the gutter may not align.
Scaling should be performed uniformly by applying the same percentage horizontally and vertically, or it will introduce undesirable visual effects. For example, a circle will become an ellipse.