Image Capture - Files very large

You can try opening the PDF with Preview. File -> Export then click the Quartz Filter drop down list and

select 'Reduce File Size' and save the new PDF. Image quality is likely to be reduced but if you are happy with the results, then you now have a workaround.

***You possibly have these options available when initially saving the scan to a PDF.

The people sending you smaller PDF's are unlikely to be scanning content to create a PDF. They generate those PDF's based on source digital documents. When you scan to PDF each page is a full page image which takes up considerably more space.

PDFs are written in PostScript, which is plain text code that instructs printers how to precisely render the document. Adobe invented PostScript and a PDF is really just a compressed PostScript file with a bunch of additional features including security options. Mac's use a technology called DisplayPDF to render the screen. This is why macOS can create PDF's so easily without 3rd party software.

Since PostScript is mostly text (w/potential images) it will compress much more than a PDF comprised of page after page entirely filled with a large image from the scanner.

You may see better compression with 3rd party tools such as Adobe Acrobat, etc. Depending on the source of the scans, you might make use of OCR (Optical Character Recognition) software. If you are scanning mostly text, OCR allows you to extract just the text which you can page layout and format then save as a PDF. You could alternatively create eBooks as well.

When scanning in full pages, getting several MB per page with lots of detail on it is common. It's the same, basically, as taking a photograph of the page with a digital camera, such photographs will be several MB (or more).

From the original application, if one prints to postscript and then converts from postscript to pdf or jpeg, the file size will be much, much smaller. Or print to pdf. Taking a photograph (or scanning, which is the same thing) uses much more space.

I have found that reducing the size of PDFs in Preview or Adobe Acrobat often leads to unacceptable loss of detail, although Acrobat is better than Preview is at this.

The best I have seen is with PDF Expert, a third party paid application. For me it has reduced PDF files by a factor of 100x (e.g. 100 MB => 1 MB) with no visible loss of crispness.

Do you have any control of the scanning parameters? If you do you can reduce the PPI to around 144 or 72 and see if the size is better and the images are acceptable.

There's no point scanning anything to a PDF.

A PDF is a container file that can hold raster images, vector images, fonts, text and other media. All you're doing stuffing a raster image into a PDF file.

It would be much better to scan to a JPEG and set the quality level down to a point where the size comes out smaller, but not so low that it's a highly pixelated piece of junk.

FWIW, 100dpi is smaller than 144dpi. 😎

FWIW, normally 300dpi is the standard for quality scans.