On the free plan you can add...
100 cards (notes, images, links, etc.)
10 files (PDFs, Word, PSD, etc.)
Images can be up to 10MB each
Other files can be up to 50MB each
On the pro plan you can add...
Unlimited cards (notes, images, links, etc.)
Unlimited files (PDFs, Word, PSD, etc.)
Images can be up to 100MB each
Other files can be up to 5GB each