Thanks for the replay and info. I checked the mobi format section and I came to the following conclusion that I'm best leaving the techincal ebooks in pdf format and corrping the relavent pages to remove the margins in a pdf editor.
You are correct most convertors of pdf to kindle MOBI will remove the layout (reflow) and change the font.
According to some reports the Kindle is supposed to remove margins in pdfs.
". White margins of PDF files are automatically cropped.
Another quote from the manual. This is not necessarily true, particularly in landscape mode. I had some books that retained their margins in landscape, while others lost it and the print went right to the edge. I also had some files created as .pdf that retained their margins even in portrait. In one dictionary, the portrait mode showed print considerably larger than landscape. Portrait went to the edge, landscape retained the margins and indented the entire page. "
(
www.amazon.com/review/R2F12TROV3ANLW)