Review Board supports attaching and reviewing arbitrary files.
Any file can be attached. Source code outside a repository, test data,

logs, images, PDFs, Word documents, zip files, binary blobs, whatever.

Review UIs
Some types of files have special review UIs, such as Markdown files

and images. They'll also be shown with thumbnails previewing the file.
Using Power Pack, PDF documents can be reviewed and even diffed.
Custom-built extensions can provide new review UIs for the specific files

you need to work with.

Reviewing file attachments
For files that can be reviewed within Review Board, simply click the

thumbnail. Or hover and click Review. You'll be able to leave

comments right on the file, whether text, image, or PDF.
For files Review Board can't display, hover over the thumbnail and

click Download to download the file, or Comment to leave a comment.

Clicked on every one of the files.