FormKeep natively supports uploading and storing files, see our advanced guide - How to Upload Files on setting that up.
When a user uploads a file, you will get the link to that file in your form submission.
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.