How to import a pdf in libreoffice? under ubuntu, all pages are blank

If you make sure the scan is 1 page per pdf, you can open en edit it with Gimp.

As long there is no text-recognition done by the scanning software, libre-office or open-office will never be able to do editing.

A simple scan gives you a image (build up by pixels) => editable by gimp

Afterwards it's possible to do text-recognition => editable by libre-office or open-office

It's also possible to make a vector drawing from the image => editable by inkscape

In LibreOffice Writer windows go to: Insert -> Object -> OLE Object

A popup window will appear. Ensure that the "Create new" radio button is selected. In the listbox below select "Other objects". Unfortunately this entry isn't available in the Linux edition of LibreOffice.

A new popup window will appear; "Insert object". Select the "Create from file" radio button, select the object type (Adobe Acrobat Document, etc) in the near listbox. If you don't want to see a small preview of the document mark the "Show as an icon" checkbox.