A print interior full of photographs can reach a size that is slow to upload and slower to re-upload when you spot a typo. Compression reduces that weight, and the only question worth asking is what it costs you in quality.
Almost all of a large PDF is images. Compression works by re-encoding them at a lower quality setting, which is a genuine and permanent loss of information. Text, vectors and fonts are already compact and are not where the savings come from.
That means the trade is real and it is one-directional. A file compressed too far cannot be restored, so the honest workflow is to keep the original, compress a copy, and look at the result at full size before you decide.
It can, because the saving comes from re-encoding images at lower quality. Modest compression is usually invisible in print; aggressive compression shows first as soft edges and blotchy areas in photographs.
It was probably already efficient, or it is mostly text and vectors rather than photographs. There is little to reclaim in a document that contains no large images.
Only problems caused by size. A file rejected for page dimensions, missing fonts or margins will be rejected at any size; those need a formatter or a layout fix.