NOTE: Make sure you download all PDFs and images displayed on your current website before migrating content over. If you simply copy and paste HTML code from your old website to your new website, the PDFs and images will still be linked to your old website.
How to link to a PDF on a page
When including links to PDFs on pages, DO NOT upload them to Drupal.
Instead, store your PDFs in Google Drive (or a similar system) and link to them from there. If using Google Drive, make sure Link Sharing is turned on for each document. You can do this by right-clicking on the document and selecting "Get shareable link" from the drop-down menu.
Turn link sharing on, and use the presented URL when creating a link on your webpage.