Declaration of the National Anti-Slavery Convention was written by William Lloyd Garrison for the formation of the American Anti-Slavery Society in December 1833. It was reprinted in The Liberator on December 14, 1833. It is now available in the Public Domain.
You can also use the Optimized PDF option to crank the size down in very targeted ways. Given the big before/after shift, the culprit is probably the image you're embedding. Try exporting that file in various formats (PDF, jpg, png) and then import it.
Using the Save option may not be your best bet for reduced file size. PDF formats use incremental updates when saved, adding only changes and new material, permitting the Save function to complete quickly. Using Save As rewrites the entire document and can represent a substantial size reduction in some cases.