This is a web-application to watch slides or pictures in a synchronized way together with other people: everyone who surfs to the same SlightSync URL will get to see the same slide at the same time. You can be in control of the slide show yourself, share control with others or temporarily grant control to others.
With the 'sign in' button on the top right of the page you can sign into your Google account. But this is only necessary if you want to convert a PDF to an image album, you do not need to be signed in to watch slides.
If you start from a PDF file, the individual pages will be converted to images (JPGs) and uploaded to your Picasa account. Then, these uploaded images will be used for the shared slide show.
The second choice is to start from an existing Picasa album (either your own or someone else's) and use the images therein for the slide show. Instead of doing a synchronized presentation, you could also look at vacation photos with your family.
You can also install a tool on your computer to convert a PDF to images and to upload them to a Picasa account: SlightUpload. It may come in handy if the web-based converter has problems with the PDF file.
Here you can see SlightSync in action. But remember that everyone browsing to these URLs gets to see the same thing, so don't be alarmed if the page suddenly changes. You can switch to a local, unsynchronized viewing mode by pressing 's'.
Everyone using 'master' mode can always change the page number, in 'slave' mode you can follow what the masters are doing. For a quick test, open the master URL in two browser windows and when you make a change in one, the other should follow.