Webscrape error for download URLs

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I encountered an error while scraping a University Publications page (https://cespubs.uaf.edu/publications/?&s=*&pt=*&cat=2). It had download links to PDF files like - https://cespubs.uaf.edu/index.php/download_file/1573/
This link does not re-direct to a .pdf URL, but instead downloads the file directly to the local PC. It gave some sort of Invalid URL or Invalid extension error while I webscraping the page. I'll try to reproduce the error and update it here shortly.