Sign into your Speaker Deck account and click Upload a deck at the top of the page. You will be presented with a form to upload a PDF.

Click the browse and select link and a file browser will open. Select the PDF you want to upload and then click Ok.

When the PDF has finished uploading you will see a Save this deck button. Optionally enter a name and description and select a category for the deck and then click Save this deck

Check this article for recommended dimensions for best quality: https://help.speakerdeck.com/help/why-does-my-deck-look-low-quality