Which browser you are using?
In the IE browser, run below PowerShell to let IE browser always download pdf files. Then use Adobe reader to open pdf files.
$webApplication = Get-SPWebApplication("URL") $webApplication.AllowedInlineDownloadedMimeTypes.Remove("application/pdf") $webApplication.Update()
In the Google Chrome browser, go to Settings -> Private and security -> Site settings -> PDF documents -> Download PDFs. When you click PDF file, in the pop-up box that pops up in the lower left corner of the browser, set Always open in Adobe reader, then open the PDF file.
In the Edge browser, go to Settings -> Cookies and site permissions -> PDF documents -> Enable Always download PDF files. When you click PDF file, the browser default downloads PDF file, then you click open file.
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