Click into the article you want to update. If it is a PDF article it will show a preview of the PDF.

In the bottom right there is a button 'edit article' click this and a pop up will show allowing you to drag and drop the new PDF or click to open the file picker.

Select your new PDF which will then be uploaded.

Click 'post upload' and the preview will be updated.

When you are done click 'Save article' also in the bottom right in order to update that content in the mobile app.