What is a Trimbox?
A PDF describes the content and appearance of one or more pages, and also contains a definition of the physical size of those pages. There can be up to five different definitions in a PDF relating to the size of its pages: MediaBox, CropBox, BleedBox, TrimBox and ArtBox. These are called the page boxes or boundary boxes. PDF boxes describe the page geometry of PDF. They are used frequently in the printing industry.
The TrimBox has a special meaning when it comes to printing a PDF. This is the most important pagebox as the Trimbox defines the intended dimensions of the finished page after trimming. If it is not set, the Trimbox will equal the CropBox. Aside from the Artbox, the size of the Trimbox is usually the smallest pagebox. In order to view a PDF at it's finished (trim) size, set the CropBox to match the TrimBox. Some systems do this by default.
Read more about the other PDF boxes in the following article: What are PDF Boxes?