Hi,
Thank you for contacting Microsoft Community.
I understand that you want to compress a PDF file.
I will certainly help you with this.
I would suggest you to go through the below link and check if it helps:
Zip and unzip files
http://windows.microsoft.com/en-IN/windows-8/zip-unzip-files
I hope this helps. Please let us know the results. Feel free to write us back for any further assistance with Windows, we’ll be glad to assist you.