We describe how container files in Portable Document Format (PDF) can be modified to inject modifiable pads for use in a steganographic file system. This produces a footprint that is less conspicuous than a randomized disk volume or disk image file. The PDF standard can be exploited to inject a modifiable segment into a file, without affecting the file’s interpretation or validity; these can be disguised as high-entropy segments that commonly occur in PDF files. We show two methods of achieving this embedding.