Besides the GUI-based tools mentioned in the other answers, there are a few command line tools which can transform the original PDF source code into a different representation which lets you inspect the (now modified file) with a text editor. All of the tools below work on Linux, Mac OS X, other Unix systems or Windows.
qpdf (my favorite)
Use qpdf to uncompress (most) object’s streams and also dissect ObjStm objects into individual indirect objects:
qpdf --qdf --object-streams=disable orig.pdf uncompressed-qpdf.pdf
qpdf describes itself as a tool that does “structural, content-preserving transformations on PDF files”.
Then just open + inspect the uncompressed-qpdf.pdf file in your favorite text editor. Most of the previously compressed (and hence, binary) bytes will now be plain text.
mutool
There is also the mutool command line tool which comes bundled with the MuPDF PDF viewer (which is a sister product to Ghostscript, made by the same company, Artifex). The following command does also uncompress streams and makes them more easy to inspect through a text editor:
mutool clean -d orig.pdf uncompressed-mutool.pdf
podofouncompress
PoDoFo is an FreeSoftware/OpenSource library to work with the PDF format and it includes a few command line tools, including podofouncompress. Use it like this to uncompress PDF streams:
podofouncompress orig.pdf uncompressed-podofo.pdf
peepdf.py
PeePDF is a Python-based tool which helps you to explore PDF files. Its original purpose was for research and dissection of PDF-based malware, but I find it useful also to investigate the structure of completely benign PDF files.
It can be used interactively to “browse” the objects and streams contained in a PDF.
I’ll not give a usage example here, but only a link to its documentation:
pdfid.py and pdf-parser.py
pdfid.py and pdf-parser.py are two PDF tools by Didier Stevens written in Python.
Their background is also to help explore malicious PDFs — but I also find it useful to analyze the structure and contents of benign PDF files.
Here is an example how I would extract the uncompressed stream of PDF object no. 5 into a *.dump file:
pdf-parser.py -o 5 -f -d obj5.dump my.pdf
Final notes
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Please note that some binary parts inside a PDF are not necessarily uncompressible (or decode-able into human readable ASCII code), because they are embedded and used in their native format inside PDFs. Such PDF parts are JPEG images, fonts or ICC color profiles.
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If you compare above tools and the command line examples given, you will discover that they do NOT all produce identical outputs. The effort of comparing them for their differences in itself can help you to better understand the nature of the PDF syntax and file format.