Select a simple PDF file and try to open it with NotePad or UltraEdit (Try to switch to text mode not hex mode). You will see more detail about the PDF file.
For example, save the following text into a *.pdf and open it you will see "Hello World" in the bottom of the page:
%PDF-1.4
1 0 obj
<< /Type /Catalog
/Outlines 2 0 R
/Pages 3 0 R
>>
endobj
2 0 obj
<< /Type /Outlines
/Count 0
>>
endobj
3 0 obj
<< /Type /Pages
/Kids [4 0 R]
/Count 1
>>
endobj
4 0 obj
<< /Type /Page
/Parent 3 0 R
/MediaBox [0 0 612 792]
/Contents 5 0 R
/Resources << /ProcSet 6 0 R
/Font << /F1 7 0 R >>
>>
>>
endobj
5 0 obj
<< /Length 73 >>
stream
BT
/F1 24 Tf
100 100 Td
(Hello World) Tj
ET
endstream
endobj
6 0 obj
[/PDF /Text]
endobj
7 0 obj
<< /Type /Font
/Subtype /Type1
/Name /F1
/BaseFont /Helvetica
/Encoding /MacRomanEncoding
>>
endobj
xref
0 8
0000000000 65535 f
0000000009 00000 n
0000000074 00000 n
0000000120 00000 n
0000000179 00000 n
0000000364 00000 n
0000000466 00000 n
0000000496 00000 n
trailer
<< /Size 8
/Root 1 0 R
>>
startxref
625
%%EOF