Convert a html book to a pdf file

1) Use ghostscript to convert the PDF to a monochrome PostScript file using the psmono device:

gs -q -sDEVICE=psmono -o mono.ps input.pdf

2) Then convert the monochrome PostScript back to PDF:

ps2pdf mono.ps

EDIT: The psmono device creates a 1-bit half-tone image which is apparently not what you want. I couldn't find a way to specify a threshold using ghostscript, so I resorted to imagemagick. convert internally uses ghostscript to convert the PDF. It then applies the threshold filtering to produce a 1-bit image and uses ghostscript again to create a PDF. Since convert uses a resolution of 75DPI by default, which might not match your actual resolution, you can provide the density argument. And experiment with the threshold setting. The optimal values highly depend on the input file.

convert -density 150 -threshold 50% input.pdf output.pdf

Maybe is a long shot, but I use pdflatex. I create a file (with a script or whatever) of the style:

\documentclass{report}
\usepackage{graphicx}
\begin{document}
\includegraphics[width=0.95\textwidth]{img000}\par
\includegraphics[width=0.95\textwidth]{img001}\par

[...]

\includegraphics[width=0.95\textwidth]{img200}\par
\end{document}

And then run it with pdflatex file. The composition is fast (and you can easily --- if you know LaTeX --- change shape and position of the images, adding captions, etc...)

The problem is that the file is normally way big; I tested with 200 jpg of 500K+ --- the run took around 7 seconds on my i5/16G ram and gave a 800Mbyte PDF. I am trying to reduce its size by using

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=lowres.pdf file.pdf 

...and it has been running 8 minutes, but it has not used a lot of RAM. I cannot comment on the compression because gs is smarter than me and discovered I was using the same image 200 times so compressed the thing to a 50k PDF... which is clearly not real.