You seem to be assuming that PFE’s stock will continue to fall between now and the deal closing...
Agreed, it's just an assumption based on nothing more than the general observation that the acquirer's PPS usually declines.
On Friday I was listening to an analyst on CNBC describe the deal as (and I'm paraphrasing from memory here) "two train wrecks merging into one train wreck." :-)
Pfizer is now the owner of two promising ALZ drugs that are both in phase III trials.. It'll have control of Elan's bapineuzumab and if you recall it has Dimebon in phase III also.
I think it gained Dimebon from Medivation last year.
Looking at Elan's stock drop 10% today it seems like the street thinks PFE will push Dimebon.