How to open PDF raw?
Looking at the raw code of PDFs will not serve you much unless you also have an idea about its internal structure. You should get yourself a copy of the official PDF reference (download PDF), and you should have read some introductionary article such as this [gone] or this to begin with.
Even after such a preparation, you'll not discover much useful when staring at the raw code. Because PDFs usually will contain parts which are "filtered" (that means: compressed).
How to look at the real PDF source behind the 'raw' binary parts
Jay Birkenbilt's qpdf is a very useful commandline tool (available for Linux, Mac OSX and as source code, under the open source Artistic License), which can unpack most filtered content and re-organize the internal structure in a way that gives you much more insight into it (all objects are numerically ordered, etc.). The commandline to achieve this is:
qpdf --qdf original.pdf unpacked.pdf
Another useful and free tool (GPL licensed, but Linux-only AFAIK) to look into PDFs is of course PDFEdit. This one even comes with a GUI (if you prefer that), while still allowing you access to the internal structure and "raw" PDF code.
If the purpose is just to look into the file, then any simple text editor will do, ex, Notepad. PDF is just a text based format, including embedded content byte streams. Raw PDF looks like this:
>>
/Border [0 0 0]
/Rect [121.02 332.48 363.24 343.64]
/StructParent 1321
/Subtype /Link
/Type /Annot
>>
endobj
64579 0 obj
<<
/Filter /FlateDecode
/Length 5771
>>
stream
Ũn0x/�+�}�ǹ����\֛ bYO�5[��X��W��L��(�������V�A3�C���������u큋_�a��ךm2N�6� ��A��8
�d���NQ⺢GI��G�[��)�̉Y��R�y{R����&�&�;��g�k1���ҋeTC�(W��`���*��(;�AEc<= mnZ+��|T��v
�.��зe�aޞ��V4�b���L����k�Oj.ֿ�y�����kc|I�� ��C�0��Hf�7d�/�z���m��o��A��B��IJ�%�.
!�%f�б���&�ޒ�4Ύ7�l�3���3`�
endstream
endobj
64580 0 obj
<<
/Border [0 0 0]
/Dest <E4AE7DD2769553EF1668>
/Rect [219 648.5 256.8 659.66]
/StructParent 1323
/Subtype /Link
/Type /Annot
>>
What you see are basic COS objects like name, dictionary, stream and so on. All objects are described in PDF 32000 standard, see section 7.3 Objects.