As far as I know there is no way to do this natively as you import them. However, a second best solution would be to search for the PDFs that you want to be tagged with the same tag and then mass tag them all. So for example, search for invoice pdfs and then add the tag invoice, then repeat for others. If you let me know what version of Evernote you are using I can do some screenshots for you. (by version I mean are you using the Legacy desktop app, web app, or the new v10 desktop app). Mass tagging can only be done on desktop and/or web at the moment, not mobile apps.