Converting PDFs is a variable disaster depending on content.
1) Mostly Image: Really a waste of effort. Best to trim margins and resample/ resize. Do not convert.
2) Text Content: Don't convert directly. Somehow copy/paste or export to Word or LO Writer, Save As DocX, make an epub2 in Calibre and convert the epub2 to azw3.
On linux any decent PDF viewer will allow export. On Windows use Ghostview.
As last resort you can import an image based PDF into GIMP as a layer per page, make an mng in reverse layer order and then ImageMagick to adjust (enable PDF write in security). Then transfer via USB (sideload) as a PDF.
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