How to ask a question about conversion problems
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Posting relevant settings and conversion logs is the main thing
FWIW - I don't do conversions of CBx to PDF, but I just tried converting a couple of CBZ's to PDF, and I didn't see anything like you describe - images were resized consistently, albeit in black and white. That was on 2.85.1 and I wouldn't have thought CBR and CBZ would make a difference. What format are the images in the CBR? Mine were jpegs.
BR