There is no catch as there are no iframes allowed for use in any eBay listing.
What you are seeing in some listings is how the server has chosen to render the listing according to the browser.
In some it ...
The URL shown on the question is using /pub. This means that the Google Document was published. I think that it's fair to assume that you published it as HTML and this setting, at least for Google Documents, hasn't "something that can be added to the src-URL" to show the document as A4 page sized document.
One alternative is that you publish your document ...
To do this, you need to combine the src strings from the embed codes. So if you want to display these two calendars:
<iframe src="https://calendar.google.com/calendar/embed?src=555555555555555555555%40group.calendar.google.com" style="border: 0" width="800" height="600" frameborder="0" scrolling="no"></iframe>
This is accomplished with the CSS rule
Note that Opera uses "-webkit-user-select" because this CSS is still experimental.
Extensions like RightToClick on Firefox override this. A userscript that overrides this CSS rule would also work.