There are plenty of online and offline tools that allow you to compress PDFs and make them easier to share in different ways. However, most of these free online compressors do not allow any level of compression customization, which means you don't have any control over the extent to which the file is reduced.
Keep in mind that security is equally important when working with online tools. With offline PDF compressors, such as PDF Extra, you work locally on your desktop and so security is not an issue. With PDF Extra's compression, you can also decide how much reduction you want to apply to the embedded images.