[markony]

My experience on the Touch Lux 3 is that a djvu version of a pdf file opens much more quickly and is smaller in size. You can convert at various sites on line. I generally download djvu files rather than pdf from archive.org because of this better response.
Alternatively use k2pdfopt to "improve" the response time of the pdf. I've taken a 160MB pdf, processed through k2pdfopt default settings and the opening and paging time was vastly improved. This is a very helpful piece of software.
Again an alternative is koreader, which has a good pdf range of options (uses a version of k2pdfopt built in), with a nightmode too!
However pdf's seem to be encoded in various forms/quality and not all can be guaranteed to work well on ereaders via the above methods.