See basically, the PDF you might have, would have consisted mostly of text with very few pictures. ASCII packs very well. If you really need to know how compression works, read up here: http://en.wikipedia.org/wiki/Data_compression
Actually thats possible! Just make a text file consisting of only a single char repeated billions of times, till the file size reaches 4.8 GB. Then try to compress it. It will have a *very* good compression ration. Such files are called decompression bombs.