Preview size of the PDF files in the article would vary for browsers. However, you can configure the desired height and width of the PDF file.
Solution
Navigate to Settings (
) > Knowledge base site > Site customization > Custom CSS & JavaScript in the Knowledge base portal. 
From the left navigation pane, click on the CSS tab and paste the following CSS snippet:
object {
height: 500px !important;
width: 500px !important;
}Update the desired height and weight as per your requirement.
Click Save.

Outcome
