I have a page called page-about-us.php within this page I have a link to another page which will link to accreditations.php. What is the correct way to do this through my page-about-us.php file?
Relative linking is one of your best friends in the basics of web.
<a href="accreditations.php">Relative Link</a>
<a href="://example.com/accreditations.php">Absolute Link</a>
The first link will always use the current directory/location and go from here. If you were on a page such as http://example.com/inner/directory/page-about-us.php then clicked on it, it would take you to http://example.com/inner/directory/accreditations.php.
As for the absolute link, the link will always go to the specified URL regardless of current location.
You should use relative linking, this is guaranteed not to break if you move your website to a different domain name.
If I'm understanding your question, you are looking to put a link on one page to take you to another page.
If you use the pagepress editor that uses default wordpress you can achieve it by looking for the link icon which is a string.
If what I look for is to edit the php file directly I think you can put it as you normally would.
Per the official documentation of WordPress (Found here: https://codex.wordpress.org/Linking_Posts_Pages_and_Categories#Dynamic_Linking_in_Templates)--
Whether you use permalinks or not, in templates you can link to pages or posts dynamically by referring to its unique numerical ID
<a href="<?php echo get_permalink(ID); ?>">This is a link</a>
Hope this helps! -Rush