[pwg]

Consider archiving them as djvu (http://djvu.sourceforge.net/). One bit per pixel djvu files at 150PPI will likely become 2-5KB pages instead.

Djvu also supports a text layer just like PDF.

Note that 150PPI is barely better than FAX, so your documents will likely look 'faxed' if you ever have to output hardcopies for some reason.

[upofadown]

Djvu is patented. As a result it is very possible that is will never achieve enough critical mass to be suitable for long term archiving.