Your "polyglot" is not a valid PDF file. A canonical PDF file starts with the PDF header (%PDF-1.x). However Acrobat Reader and some other PDF viewers are less strict, from the implementation notes in the PDF specification:
3.4.1, "File Header"
13. Acrobat viewers require only that the header appear somewhere within
the first 1024 bytes of the file.
A canonical PDF file ends with %%EOF in the last line. Again, Acrobat Reader is less strict:
3.4.4, "File Trailer"
18. Acrobat viewers require only that the %%EOF marker appear somewhere
within the last 1024 bytes of the file.
Thus there is some room to add a wrapper for other formats.
However, you cannot add something at the beginning or the end with pdfTeX.
But something can be added inside the PDF file:
\begingroup
\pdfcompresslevel=0\relax
\immediate\pdfobj stream file{foo.bar}\relax
\endgroup
Then the contents of file foo.bar can be found uncompressed inside the PDF file.