Reduce PDF file size when exporting from InDesign

Something doesnt quite seem right here, i've literally just exported a 36pp brochure that has over 100 high res medium/large size images in it using smallest file size and that PDF was 3.6MB....on PDFx/1 full high res the pdf size was 25MB ! if you're able to send me a packaged copy via WeTransfer i would happily take a look for you!

There is an ancient out-dated way to make a pdf which does usually produce a much smaller pdf than more modern methods. To do so you must first save the document as a postscript file. To do that go to InDesign's Print dialog window and at the top where it says "Printer" pull down that menu and choose "PostScript® File". Then make all of the choices that you would if you were about to print the document in regard to page size, crops, bleed and individual pages/spreads. Then click on "Save" in the lower right corner and save the postscript file to your desktop or other convenient location. Now go into your Acrobat application folder and launch the Acrobat Distiller app that will be there (you might want to brush it off for cobwebs as it probably has been sitting there gathering dust). When you launch Distiller you will see at the top of the dialog window a pull-down menu called "Default Settings" with options for file size. Pull that down and choose "Smallest File Size". Now drag the postscript file right onto the Distiller dialog window. It should begin distilling the file. When it is finished it will leave the pdf in the same location as the postscript file. This pdf should be significantly smaller than what you have been getting.

Now, as I said, this method is not approved, modern or even tolerated by many. So, if you tell anyone I told you about it, I will deny it.

Try using Acrobat, Save-as-other> Optimized PDF. Select all clean up and discard options (unless there is something you need to keep). If the PDF is still too large, there may be some complex vector or pattern information that is unaffected by downsampling, if so, you will need to replace this art with a raster version to make a small PDF. This PDF will be for on-screen proofing only (not for printing). You can use Acrobat to extract the PDF into 16 individual pages, if you are lucky, one page will be much larger than the others, helping to identify the page with complex vectors.

Also, you can share a link to a high res PDF directly from Acrobat (buttons upper right corner), although I see you are trying to avoid that.

The size is normal for a high res file. Make a low res file or stop trying to email it.  Emailing PDFs is never a good plan, and emailing design work is rarely possible. I've never found any recipient to have a problem with wetransfer, but your mileage may vary.