I found a similar thread that gave some great pointers, but still doesn't solve my issue.
My boss asked me to create a corporate presentation, and I had the unfortunate idea of doing it all in photoshop (I figured it would allow greater flexibility than the usual powerpoint).
I created 24 slides at 3000x2000px and 300dpi, for a total size of 69Mb.
Now obviously this is not ok for a presentation that must be sent via email to multiple clients, so i decided to reduce its size using the technique described in the thread above.
It all worked great and the file size is now down to 4Mb, only problem is that the text is starting to blur. Since it appears that text makes up for only about 3% of the total file size (it's the images that are bloating up the Mb count), I was wondering if it's possible to reduce the image quality without "touching" the text layers.
In powerpoint you can have a bad quality background image and write perfectly sharp text on in. Even at 640x480 text looks good in powerpoint. On the contrary, if I lower the res in photoshop, the text turns crappy.
So is there a way to add hi res text to a low res "background" image in photoshop?
UPDATE: after a long afternoon of work, and thanks to John's help, my presentation is now perfect, so I thought I'd give back to the community an share a couple more things I've learned working on this project.
As you can see, like John said, if you have faux styles applied, the text is automatically rasterized and this creates a lot of artifacts during the conversion. On the contrary, if there are no faux styles, the text stays in its vector form and, even when you reduce the image quality a lot (see the picture on the right side of the image), the characters remain tack sharp.
So I went ahead and re-formatted all the text boxes in my presentation. Again something went wrong see here. As you can see, sometimes the PDF version screws up your text. As silly as this may sound, I found that this only happens when you write too much text in a single box. All text longer than 6 or 7 lines got screwed up. Splitting it into two different text boxes with 3 lines each fixed this. The same problem happened on multiple slides, and the same solution worked to correct the problem on all of them.
Last but not least, I managed to decrease the total file size some more by going to file->Automate->PDF presentation (not sure about the exact translation cause my photoshop is in italian) instead of the usual file->Save As->Photoshop PDF. Try for yourself, with the same compression values the first option yields smaller files (some 30-50% smaller!!). So I guess there's some hidden parameter that the "Automate" command sets somewhere that the "Save As" does not.
I now have a 2,8Mb presentation with razor sharp text and low-res images (still looking good ad 100% zoom, artifacts start showing only at 120%). Thank you stackexchange! :)