Fortunately or unfortunately, spammers are good at convincing people to click links in emails. How do they do it?
Personalize it - Include the name of the sender (assuming the recipient knows the sender.) Use the recipient's name. This proves it is not a generic email.
Spoof your sending address - Make it appear the email came directly from the sender (the user of your app) and not from firstname.lastname@example.org.
Don't make the link seem scary - Use a picture instead of a hyperlink, and don't use the word "download". The 5 o'clock news tells people to avoid "links" or "downloads" in emails, and many users consider these different from "pictures" or "buttons".
Explain the benefit of clicking - Again, avoid scary words like "download" or "link", but write a nice paragraph with something along the times of "Click below to see [Your-Friend's-Shared-Material]!"
Make an immediate call to action - This is big in phishing schemes. Don't allow the user time to think about whether it's a good idea or not, press them to make a rash judgement. Say that it expires after a certain time.
Another success story from chain emails:
- Play on user's superstitions - Tell them something negative will happen if they don't click on your link. Or tell them something good will happen. It's best if it is intangible, such as having good or bad luck for a certain length of time. (I don't recommend you actually copy this, but it's worthwhile to note that it has had success.)
With this said, I personally would never download anything in an email I wasn't specifically expecting. If I really thought I may want it, I would call the sender and confirm they sent it. If I don't have the sender's phone number, it's getting deleted. You will likely have users like this, so if getting 100% to click your link is important you may want to consider another method.