I need to edit the text and then make a new pdf file. The pdf file contains no images.
Open the document in your normal PDF viewer (Sumatra PDF works well in Windows) and highlight the text you want to copy. Ctrl-C (as usual). Then open a new document in LO Writer, and Ctrl-V (as usual). Works for me (LO 6.0.7.3 on Ubuntu 18.04).
If there are minor changes to do, open it in LibreOffice-draw, do the changes and export to pdf.