asciidoc not rendering pdf image includes

I am using asciidoc to document a project. In some of the files, I need to include graphics including jpg, bmp, and pdf. The first two render fine when included:

[#img-someJPGsimage]
.This is some figure notes
image::artifacts/someimage.jpg[altText]

However, when I try this with pdf, all I get is a 'broken image' icon and the alt text.

[#img-somePDFimage]
.Some notes for this image
image::artifacts/someOtherImage.pdf[altText]

Thoughts?