The error message you're seeing, "Refused to display 'https://docs.google.com/viewer?url=http%3A%2F%2Fleaderpaper.com%2Fleader%2Fspecs%2F10-openend.pdf' in a frame because it set 'X-Frame-Options' to 'SAMEORIGIN'", indicates that the website you're trying to embed in an iframe (the Google Docs PDF viewer) has set the X-Frame-Options header to 'SAMEORIGIN'. This header prevents the page from being displayed in an iframe on a different domain for security reasons.
This behavior is intended to prevent clickjacking attacks, where a malicious website could try to trick users into performing actions on a different site without their knowledge.
Your workaround of opening the PDF in a new tab with the _blank target attribute is a common solution to this issue. When you open the PDF in a new tab, you're essentially loading it in a separate browsing context, so the same-origin policy and X-Frame-Options header restrictions do not apply.
So, you've done what you can to work around the issue within the constraints of how the website you're embedding handles iframe embedding. Opening the PDF in a new tab is a practical solution when you encounter X-Frame-Options restrictions, and it's generally user-friendly since it ensures the PDF can be viewed without issues in most browsers.