Scholarly reading often involves engaging with various supplementary materials beyond PDFs to support understanding. In practice, scholars frequently incorporate such external materials into their reading workflow through annotation. However, most existing PDF annotation tools support only a limited range of media types for embedding annotations in PDF documents. This paper investigates cross-media annotation as a design space for augmenting academic reading. We present a design exploration of a cross-media annotation tool that allows scholars to easily link PDF content with other documents and materials such as audio, video or web pages. The proposed design has the potential to enrich reading practices and enable scholars to guide and support other researchers' reading experiences.