Hi Arthur,
We regret for the inconvenience caused.
Please download the Help documentation setup for Syncfusion Essential Studio setup v5.1.0.51 from the below link.
https://s3.amazonaws.com/files2.syncfusion.com/Installs/v5.1.0.51/syncfusionessentialstudiohelpsetup51.zip
Now, you can able to install the Syncfusion Essential Studio Help setup in your machine by extracting the downloaded zip file.
Please let us know if you have any quries.
Regards,Kathiresan s.