Suppose your personal website is hosted in a Github page as follows:
https://username.github.io
The repository should be name as username.github.io. If you have a pdf file named document.pdf and you place it in the directory folder then you should be able to open it directly in the browser through the following link:
https://username.github.io/folder/document.pdf
To allow the user to open the pdf in a new window in the browser, you may use the following HTML, where “PDF” points to the link:
<a href="https://stackoverflow.com/questions/30745981/username.github.io/folder/document.pdf" target="_blank">PDF.</a>