Reason of can't download a PDF file on your mobile:

• Try right-clicking the file's link and opening it in a new tab or with a different application, such as Adobe Acrobat DC Reader. … Toggle 'Display PDF in browser' on or off in Adobe Reader by going to Edit > Preferences > Internet. Then, to ensure that the configuration change takes place, close Adobe Reader.
• Clear your cache as well as the Google Play store's app data. After that, restart your device and go to the Google Play Store to begin installing apps. If you recently installed a Google Play Store update, uninstall it, restart your phone, and try it again; it should work. Look for data with a Restricted Background.

Read More: How should I recover the PDF files in WhatsApp, that were not downloaded?