From The Document Foundation Wiki
- create or open your document in LibreOffice. All document types available in LibreOffice are supported;
- go to ▸ ;
- check to insert the ODF editable file into the PDF;
- you can also change the other parameters such as the image resolution. Do not forget that large images make for large documents; and
- click the Export button and complete the save operation (choice of folder and filename).