If you find you have a multi-page PDF with huge margins on every page, you can use Preview to crop those margins down.

With the PDF open in Preview, make sure that the sidebar is visible (in the View menu) and set it to display thumbnails using the button at the bottom.

Change to the select tool using the toolbar button, the Tools menu or by pressing Command-3. Drag a box around the region you want to keep, then click in the sidebar and press Command-A to select all pages.

If you have some pages that are a different orientation (i.e. landscape), you can hold Command and click on these pages to de-select them.

Finally, choose Crop from the Tools menu, or just press Command-K.