If you have the Google Drive integration enabled, you can import instantaneously without leaving Slab! Just click the drop-down arrow next to Create a new post and select Import from Google Drive.
You can then search for the name of the file you want to import. It'll be instantly uploaded to Slab!
Note that only Google Docs and .docx files are supported right now — you won't be able to import PDFs or Sheets from your Drive.