Currently there are two optional ways to compress a PDF:
1) Online Foxit PDF Editor: If you’re using it but you failed, please try to compress again. Sometimes it could be network glitch. Please try again: https://pdfonline.foxit.com/
2) PDF Optimizer feature in Foxit PDF Editor software: Foxit PDF Editor is free to try out for 14 days. You may download it from Foxit website:
https://www.foxit.com/downloads/
After the installation, please do the followings:
For Foxit PDF Editor versions prior to V10,please go to File, select Reduce File Size or PDF Optimizer for compressing document.
(Note: Reduce File Size is the part of PDF Optimizer function. It utilizes the default settings of PDF Optimizer.) 
For Foxit PDF Editor v10 and later, please go to File>PDF Optimizer, select Reduce File Size or Advanced Optimization for compressing PDF file.
(Note: Reduce File Size is the part of Advanced Optimization function. It utilizes the default settings of Advanced Optimization.)
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Relative Topics:
Advanced Optimization vs. Reduce File Size—which one should you use to make PDF files smaller?
Optimizing PDF Documents for File Size
More Tips for Managing PDF File Size
