Links not working, same directory as index.php

[Alaskadadabra]

Hello everyone, hi ritwick.
The live server is working so far. Very good!
The problem is that when I click on links within my "index.php", access is denied.
For example, a link to a PDF that is in the same directory as index.php.
A separate page opens with the heading "Access denied!" and below it "Error 403".
What am I doing wrong?
This also shows that something is wrong: When I reference the styles.css in my index.php to access it, the style is not applied. Only when I inline the CSS within the index.php does it work.
Styles.css is of course also in the same directory as index.php, so I assume that it is not a case of incorrect linking.
How can I fix this?
Kind regards