Apple - Measuring PDF margins, etc. with Preview?
Found it!
In Preview,
- open the Inspector (Command-I)
- Select the Crop and Rotate tab, whose icon looks like a small ruler. Change the units as required
- Now use the select area tool and the corresponding (printed) dimensions will be shown in the Inspector.
As an alternative, you can also download Free Ruler, which displays a nice ruler on screen, though requires care in both setting the ruler DPI and ensuring that the PDF is viewed at 100% size.
In macOS Mojave, this works:
- Tools > Show Inspector (Command-I)
- Click the 'ruler' tab in the inspector window, and select the units that you want
- Tools > Rectangular selection
- Drag in the pdf to make a selection, and the inspector will show the size of your selection