Copying files to tablet with Android When I copy file.pdf to my tablet with the command (in Linux - Ubuntu)
cp file.pdf /media/Archos_7/Data via USB cable and then disconnect USB and look to the Data directory, file.pdf has size 0 B, or the file is not there (e.g. file.epub). I don't know what is wrong. I copied a few *.pdf and *.avi files successfully.

Do you know where is the problem?
Does your device auto-mount its storage to the PC when connected?  On most devices you need to pull down the notification bar and click the USB notification to mount.  You should dismount the same way, after dismounting in Linux, before unplugging the device.

And, how much free space do you have?