I am sharing my experience with reducing scanned pdf file size.
I scanned a 11-page papercopy into a pdf file. I used 150 dpi for scanning. The file size is 5.06 MB. The file is too large to upload.
I split the file into two files by choosing Adobe Acrobat Reader's Print/Microsoft Print to PDF/. The individual file sizes are 1.1 MB and 1.93 MB for a total size of 3.03MB.
Next, I printed the full file choosing Adobe Acrobat Reader's Print/Microsoft Print to PDF/. The file size is 3.04 MB, which is almost the same size as the total of split file sizes.
So, looks like Adobe Acrobat Reader's Print option of Microsoft Print to PDF compresses the 150 dpi scanned pdf file by about 40%.