Most email services stop attachments above about 20 MB, and many forms accept much less. The Compress PDF tool saves the file again and makes the pictures inside smaller.
Steps
Open the Compress PDF tool and add your PDF.
Press the compress button.
Compare the new size with the old size.
Download the result and open it to check that the pages still look good.
If it is still too big, try Compress PDF to a size and type the number you need.