I have libre office and want to convert to libre office
I assume your pdf-file is a text file and you want to open the pdf-file using Writer. This is only possible when it was created in OpenOffice and the OpenOffice file was embedded.
Workaround if you NOT such a file:
Use a software which can convert a pdf-file into rft, odt, doc, docx format. example pdf-Word
You can also look for an OCR SW like ABBY_FineReader. Depending on the file i nee to convert, one or the other is better.
As both applications are not free like a free beer, I would search the web for free versions. If you find some interesting application you might be interested in sharing links here.