How can I compress this pdf file without losing readability?

If you want to maintain the content integrity of your document, I advise against OCR. I think most OCR engines would have a lot of trouble vectorizing your content, due to the heavy use of symbols, mixed use of italics, bold and normal fonts, and complex formatting.

If you simply want to make the final PDF file smaller, you could always compress it using a simple file compression utility available on your computer. For example, within Windows, you could right-click on your file and select "Send to" --> "Compressed (zipped) folder". I've found that does a fairly good job of compressing PDFs. Of course, the downside is that you'll need to uncompress it when you want to read your PDF.

Try searching for compressing PDFs in general. I personally use these:

pdftk in.pdf output out.pdf compress

Or:

pdf-compress-gray () {
    local input="${1}"
    local out="${2:-${input:r}_cg.pdf}"
    local dpi="${pdf_compress_gray_dpi:-150}"
    gs -q -dNOPAUSE -dBATCH -dSAFER -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -sProcessColorModel=DeviceGray -sColorConversionStrategy=Gray -dOverrideICC -dDownsampleColorImages=true -dDownsampleGrayImages=true -dPDFSETTINGS=/screen -dColorImageDownsampleType=/Bicubic -dColorImageResolution=$dpi -dGrayImageDownsampleType=/Bicubic -dGrayImageResolution=$dpi -dMonoImageDownsampleType=/Bicubic -dMonoImageResolution=$dpi -sOutputFile="$out" "$input"
}