Since SeaMonkey 2.53.1, installed NPAPI plugins are no longer supported in SeaMonkey, except for Adobe Flash Player. Some of the plugins that no longer load in SeaMonkey, even though they may be installed on your computer, include Java, Silverlight and Acrobat Reader.

If you need a replacement for the Acrobat Reader plugin take a look at the pdf viewer adapted for SeaMonkey by Isaac Schemm.