We recommend using Shubox.io to upload and store files to S3.
When a user uploads a file, you will get the link to that file in your form submission.
Shubox.io provides a special FormKeep tier for our customers, check out the Data Integrations tab under Account Settings to get additional information on the offer.
They have a super simple script you can copy and paste onto your website to get going, and lots of configuration to make it look and behave like you want. Check out their amazing demos.
You will need to setup an Amazon S3 account, but Shubox has some great documentation on how to set that up and get everything working together.
FormKeep does work with all other file upload services in additional to Shubox, so if you already have an account with another provider it will be easy to use FormKeep with them as well.
Finally, in the FormKeep Submission preview page, any urls that are in the form will be displayed. This includes images and pdf documents. Other url types will display as a clickable link.
Submissions marked as spam will not display this information inline, but will display the value of the url, in case you want to check it out yourself. Marking them Not Spam would allow the images/pdf to be displayed in the page.