[carpler]

I'm a newbie with ebooks and related, but there is something frustrating for me in conversion.
I've got a Nook STR and I've tried several times to convert some ebook to epub, but always with some issue.
The input format that I used was: pdf, rtf and lit, but never the epub outup had the same aspect as the original.
Why this?
Sometimes I've got pagebreak where is not, the images with used as initial letter is not alligned as the original, and so on.
Initially I thought about a pdf problem, but I did not expect the same problems for the lit format.
Why is so difficult convert files and obtain the same aspect?
Wich is the best input format to voncert in epub?
Ah: I use Calibre 0.8.17.
Many thanks for your attention