When you import a PDF the print definition is always 100PPI by default, you have to force it to 300PPI in the import dialogue to get you image at its initial size.
This said you have no control on the compression level in the PDF so exporting to PDF and re-importing will not yield the exact same image. PDF is a publishing format not a storage format(*). If you want to continue editing your image, save it as a plain image (ideally XCF, but TIFF (without JPG compression) can also do).
(*) When you use Word/LIbreOffice, to you edit the PDF or do you go back to the .DOC?
This said you have no control on the compression level in the PDF so exporting to PDF and re-importing will not yield the exact same image. PDF is a publishing format not a storage format(*). If you want to continue editing your image, save it as a plain image (ideally XCF, but TIFF (without JPG compression) can also do).
(*) When you use Word/LIbreOffice, to you edit the PDF or do you go back to the .DOC?