I have read the pdf sticky but this was written in 2011 and I'm hoping things have changed.
Besides this problem seems to have happened on one book in particular.
I just recently bought the full 7 volumes of Spike Milligan's memoirs - normally my conversions are epub and pdf however, on the first volume this refuses to handle the images correctly. The horizontal aspect is squashed in on all pictures where width is longer than height.
I have tried EVERYTHING. There is no real aspect control or resizing from what I can see - I came across the comic set up but nothing there works.
Setting output to Tablet doesn't work. Nothing appears to change this behaviour.
Having perused a few of the other volumes I failed to detect anything similar they appear to be reasonably ok although one other thing prevalent with the conversions in all cases is that the pictures / illustrations often are on page breaks and so the graphic gets cut on to two pages. This may or may not be a problem with pdf readers as I see this in the epub version but at least the view allows it to be set to flow thus keeping pictures intact.
Either way the pdf conversion of this one single volume is really strange and I don't understand why this is the only one of 7 that is affected in this way.
Also one thing in my tests were to try to convert a conversion but I don't really understand where or what is being converted. Firstly the kindle books are being picked up and in my calibre library I see a .azw3 / .epub / .pdf
however, when I convert there is no request for which version to convert. I wanted to try to convert the epub version as the aspect is correct and that would eliminate any problems perhaps originating in the original azw3 file.
Lastly I tried a plugin called prince pdf. This correctly created the images but there's no way to alter other settings the the conversion had other issues.
Attached the 2 versions showing the problem
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