Writer - Merging PDFs

Not exactly.

Inkscape imports PDFs and allows them to be ungrouped and edited.

In my case, I cut or export from Inkscape only the objects I need.
Then I rebuild the PDF format inside Writer.

It takes a while to become proficient at Inkscape, which I am not. I just use it to break apart PDFs to gain access to their parts.

I notice you have gmail. Have you tried importing PDF into Google Docs and then exporting as ODT?

This is also a method I have has some success with, but requires internet connection.