If the former owner of that MacBook Pro did not follow these Apple requirements, you will have a MacBook Pro running High Sierra with the owners Apple ID, and a Mac App Store associated with the same AppleID.
If when you first powered on the MacBook Pro, it came up with prompts for language, keyboard, and new user setup steps, etc. then ignore the first paragraph.
If your MacBook Pro was a 2012 model, then it can upgrade to Mojave, and then you can download Pages from the Mac App Store. From menu : About This Mac, if the Mac product description line has 2012 or later in parenthesis, then you can upgrade.
How to upgrade to macOS Mojave. Open that link only in Safari.
I would strongly recommend that you avoid any version of Pages from download aggregation sites, or Ebay resellers, as you don't know if the product is security compromised with badware. It also happens to get sidewise with Apple licensing too.