The latest version of Pages for Yosemite 10.10.5 is v5.5.3. Launch the Mac App Store, and pressing the option key, click on the Purchases tab. If there is an install button adjacent to Pages, click on that, let it advise that it cannot install Pages 10, and see if it offers to update your copy of Pages to v5.5.3.
Your Late-2011 iMac can upgrade as far as macOS High Sierra (10.13.6). If you choose to go that route, ensure that you have a last Time Machine backup to an external HFS+ formatted drive, and verify that your printer/scanner vendor has driver support for High Sierra. The upgrade should not touch your home folder contents, but that Time Machine backup is very good insurance.
Should you successfully upgrade to High Sierra, do the first paragraph again, and it may update your Pages, Numbers, and Keynote versions to 8.1, 6.1, and 9.1 respectively.