Sometimes your PDF is not opening properly, to fix that follow the steps below
1- Open Firefox and then click on the menu icon at the top right corner.
2- From the dropdown menu, click on “Options.”
3- After clicking on “Options,” a page will appear where you can search for applications.
4- Then search for PDF
5. Under the actions column, make sure A PDF viewer is selected
You are now all set and ready to proceed.