Here is some text. This link will open in a new tab. Compared to this link which will open in the same tab.

Here is the same treatment but with a link to a user guide. This user guide link will open in a new tab. This user guide link will open in the same tab.

On the ones that open in a new tab, this page will still be available in THIS tab, you just have to close out of the new tab you just opened to come back. Or just select this old tab here to come back to it while leaving the new one you just opened still available for future reference. 

On the ones that open in the same tab, if you hit X you just closed the only tab you have open so the whole site will close. Instead, you’ll need to hit “BACK” in your browser to return to this page. 

Same thing with buttons:

My personal take:

Open Links to Documents In a New Tab: When people finish using/reading PDF files and similar documents, I’ve read that they click the window’s close box and are less likely to use the back button. This action closes their entire browser window. 

Open Links to Internal and External Web pages In the Same Tab: Generally, links to other pages within the same website should open in the same window.
Reasoning: When new web pages open in the same browser window, users can access their browser tools (e.g., the Back button) throughout their web experience.

Possible exception: in the lessons (and other places on the site) sometimes you’re linking other pages on the site more as tangential reference pieces (e.g. talking about activity plans and then linking to a sample activity plan), rather than the next logical step in the user’s website journey… in those cases I’d say to have them open in a new tab so they can have them saved to look at later.