From The Document Foundation Wiki
- Select all the contents to be exported. You can select with the mouse of use Ctrl+A to select all the elements in the page.
- Group all elements ▸ ▸ or Ctrl+⇧ Shift+G.
- Export image with ▸ , choose a name for the file and select PNG as file type. Click .
- The Export to PNG dialog opens (PNG is a raster-graphics file format that supports lossless data compression).
- Keep the size in pixels suggested and the resolution to 96 pixels/inch. Keep all other setting as default. Click .