As an interim patch while we continue to work on the map editor, we've released a PDF viewer.
This PDF viewer is for viewing local PDFs on your machine. It does not allow you to share PDFs with other users, it just lets you bring them into the viewer so that you can reference them when playing on the sandbox table. You and a friend have the same PDF? you can both load it up and have a game.
To find it go to the tools menu and look for the PDF option. This will pop it up, click it again to remove it. It has a side panel, working search, and will remember the last page you had it open to even if you load it later.