How to make PDF smaller?

"I created the pages in Photoshop thus allowing myself to
arrange things pixel-perfectly. I exported them to BMPs (dimensions 4960x7000, A4 size in 600 dpi), then simply embeded them into a word
document and then "printed" them with Adobe Distiller to PDF files. "

This is, basically, your problem; your document consists of a large number of large images -- you're not going to reduce this a lot without some serious jiggery-pokery.

Basically, where a character would take up maybe a couple of bytes of PDF code[1] in the file, each character in the images takes up hundreds of bytes. There's no really good way to reduce these a lot.

There is a usability issue in that the resulting PDFs will rather strain machines; certainly my elderly laptop used to complain about files formatted like this and it would take ages to open them, whereas files consisting of PDF formatted text are a bit friendlier to slow/low memory machines.

I'd say if you really want to stick with this format, offer both the 600dpi and the 150dpi downloads; list one as for reading on the computer and one for printing.

[1] PDF files are actually a programming language, which can contain instructions like "print the word 'Hello' at these co-ordinates", which don't take up much space.

Katie hit the nail: your problem is in the way you created the PDF. If you place all your text on the raster images rather than in the Word document, you are limiting the power of the PDF format to a simple collection of JPEG files in one archive, and binding irreversibly the file size to the text quality.

For screen reading you can produce a 72ppi (or so) light version. Then you can offer another, higher-resolution version for those who want to print it with better quality. 200ppi should be enough for good quality images, unless you have lots of fine high contrast detail or small print text.

But if you can, you should try to use Word (or any other aplication that handles texts as text instead of as images) for text, then insert the graphics.

For images, its preferable, quality-wise, to set the final size and resolution in Photoshop rather than downsampling them with Acrobat. So if you're goinge to produce a 200ppi PDF, insert 200ppi images in your Word doc and then have Acrobat dowsample just for the 72ppi screen version.

When distiling, you can also set the image quality (JPEG compression). That's another important factor in the final PDF file size. The higher the image quality the higher the file size too. You should play a bit with this value to acheive the best balance for this particular document.

Seo

Note: Actually, PDF is not a programming language; it is a page description file format based on the PostScript programming language; PS files are programs, PDFs are structured documents which include PS instructions. But this is just a technical detail, and has not much influence on your problem.