Create readable .mobi files out of pdfs
So I've been trying to work on transferring my pdfs to the kindle 3, but I have run into some problems with formatting. When I transfer a pdf that is mostly text to a .mobi, and I then need to make the text bigger on the kindle to read the new .mobi file, it works fine. However, when I try to use calibre to make a .mobi file out of a pdf that has images throughout the text, or a pdf with multiple columns of text, the new .mobi file loses its ability to change text size when on the kindle; when I try to change the text size, the pdf image remains as a full page and won't change in size.
The only other option I am aware of is to keep them as pdfs and read them as is on the kindle. I prefer not to do this, though, as I find pdfs difficult to read and rarely worth the effort.
Is there any way to change the calibre instructions or directly alter the pdf so that the new .mobi file will have a pdf with images and will still change text size? Or a way to make a pdf with multiple columns into one stream of text? Or a way to unattach the images so that they lay above or below the text on the kindle?