Searching for "0000/01/01"[PDAT] : "3000/12/31"[PDAT]
should get you every article from the beginning of time.
Maybe the "sent to" function shown right-above the search results will let you download everything.
Alternatively, you can write a script and use the Entrez programming utility from the NCBI.
You can perform a search query using ESearch, which will return all PMID's.
Then you can use EFetch to return all data. It is explained in this book/manual:
http://www.ncbi.nlm.nih.gov/books/NBK25501/
Chapter 3 contains some example scripts which should get you started:
http://www.ncbi.nlm.nih.gov/books/NBK25498/#chapter3
You will get xml files containing the abstract and all other data.
25 million XML files...