Hi, mickael28! Welcome to the forums and congrats on getting the best eReader ever made
The first thing I would suggest doing is upgrading your ROM to the Allmine Universal, if you haven't already.
Now, for your first question; "Can you send pages from the PDF viewer on the tablet side to the eink side".
I have no idea. My eDGe still has the old hacks on it and they broke the built in copy of Documents To Go... I dunno if that is an issue with Allmine Universal or if there is a fix. I keep forgetting to find out. The Docs2Go PDF viewer is the one the eDGe uses by defaults to display PDFs on the android/tablet side and I cant remember if it lets you send individual pages to the eink side.
Now, as for where the screenshots are store:
All the Library app files are stored in the same folder. You will need to open the ES File Explorer app.
Once you have it open, click on your edges "menu" button, open "Settings", choose "File Settings" under the 'Display Settings' category (it is the third option from the top), then checkmark "Show Hidden Files". Back back out of the settings back to the main app. Depending on what folder you are in you will need to click the top left button that has a house picture. It should change to a forward slash ( / ), when it changes, click on it again.. (If it is already a forward slash, go ahead and click on it) This takes you to the root of your eDGe's storage.
Now scroll down till you find the folder "sdcard", open it up and then find the folder ".edge". Your Library apps files will be stored in the folder ".esa-content".