GDPR advice about storing personal documents

Hi there,
First post here !

I am bootstrapping a project which is about presenting your documents in a nice way instead of mailed PDF files to landlords when you're looking to rent a flat.

The thing is : I will need to ask customers to upload their pay slips, employment contracts, ID cards, tax slips... So that they can share them in a secure way (unique link with password) to landlords to check the files.