CONTENTGUARD is a free app and service which shares images (PNG, JPG) and documents (PDF, DOC, DOCX, XLS, XLSX, PPT, PPTX) with your choice of recipients, for a specified amount of time only.
After installation and signup (where you must provide your phone number), you're able to select documents, PDFs, photos or screenshots from your device.
An expiry date can be set to anything from one minute to a month.
Choose one or more recipients directly from your contacts, and they're each sent an encrypted copy of your chosen files.
After verifying their identity, they're then able to view your documents.
There's also a "Whistleblower" feature which warns you "if the recipient tries to take an unauthorized screen capture". Sounds clever, but of course once the file is displayed there are a range of ways to capture it, so we wouldn't rely on this to do anything useful.
A well-designed secure file sharing service