Add custom text at bottom of every page?

We’ve published around 200+ pages on our WordPress site and I want to add an iframe to the bottom of selected pages, or every page.

For the sake of time, is there a quick or easy way to do this job with some plugin or trick?

Depending on theme you are using there might be hooks available to output content at specific point. There is no generic convention for them, however.

The most general approach to modifying the default output of page would be modifying (or introducing) page.php template. If your base theme is upstream (not developed for specific site) and receives updates you would need to create Child Theme to not lose changes during update process.