Sharepoint - Open PDF files in adobe Reader instead in Explorer

If the PDF files are opened in the browser, then it means you have either configured the Permissive mode in Browser File Handling, or added the PDF extension to the allowed MIME type list.

To have the PDFs opened in Acrobat, you should disable the Permissive mode (and set it to Strict mode) and if it is already Strict, then remove the PDF extension from the allowed MIME type list.

# set BrowserFileHandling property as Strict
$wa = Get-SPWebApplication <web-App-URL>
$wa.BrowserFileHandling = 'Strict'
$wa.Update()

# remove PDF from MIME type list
$wa = Get-SPWebApplication <web-App-URL>
$wa.AllowedInlineDownloadedMimeTypes.Remove('application/pdf')
$wa.Update()

Read more: https://www.sharepointdiary.com/2011/04/open-pdf-document-browser-sharepoint2010.html