Most web browsers will open these very large files
at a reduced view so that the entire map is visible.
The cursor will take the shape of a magnifying glass.
Clicking on the image with the magnifying glass will
enlarge the image to full size.

Internet Explorer also opens the scans at a reduced view,
but wants you to move the cursor over the image and wait.
After a while, an icon will appear in the lower right of the image.
When you click on that icon, the image will enlarge to full size.