Use Preview: Open the PDF in Preview, click File > Export, select “Reduce File Size” under Quartz Filter, and save. Use ColorSync Utility: Open the PDF in ColorSync Utility, click File > Export, ...
These days, you don't need an app just to view PDF files. Every web browser can do that. If you sometimes create PDF files out of Office documents, you probably use Adobe's Acrobat Reader. After all, ...