I have bought several pieces of used exercise equipment (Proform Treadmill, Proform elliptical, Precor climber), and none of them came with in owner’s manual. Recently a friend of mine bought a used Horizon treadmill. She needed to adjust the belt. Of course her treadmill didn’t come with a owners manual either.
You can generally figure out how to use the equipment without an owner’s manual. But having an owners manual is nice is something goes wrong. If the belt starts to slip, or go to one side. Sometimes there are features on the control panel that you might not know how to use unless you read the manual.
But the good news is most of these manuals are online now, and you simply have to download them and print them out.

Here is where you can download the owners manual for your unit:

Horizon Owners Manuals (treadmills, ellipticals, bicycles)
(simply go to page for your unit, link to owners manual is on right side of page)

LifeStyler Owners Manuals

NordicTrack Owners Manuals (treadmills, ellipticals, bikes, skiers, steppers)

Precor Owners Manuals (climbers, cycles, ellipticals, rowers, treadmills)

Proform Owners Manuals (treadmills, ellipticals, bikes, steppers)

Weslo Owners Manuals (treadmills, ellipticals, bikes, steppers)