PDF files which are stored in a Sharepoint document library open browser automatically.
Even if you configure the advanced settings in the document library for browser-enabled documents to open in the client application, but it still opens the pdf in the web browser.
To solve this issue, you can use the below steps.
1- Go to your web browser.
2- Click on Add-ons.
3- Select "All Add-ons" from dropdown.
4- Select Adobe Reader add on and Disable it.
5- Refresh the browser.
6- Try to open the pdf file it will ask to open it in client applciation.