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I often get pdfs from engineering firms that are only 80 or 90 MBs and can contain up to 24 sheets of 36" x 24" drawings and they will print out very sharp. But when I scan 1 drawing in Photoshop and save it as a Photoshop pdf file, it can end up as 50 MB for only an 11 x 17" drawing.

How can I scan an original pencil-on-paper drawing in Photoshop and have it stay very sharp, but end up with an e-mailable file size?

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So the engineering firm's PDF probably consists of a mix of vector imagery which can be quite efficient since vector typically only stores points of interest, not the whole scene. So a simple vector image of a line on a white square might get away with a few bytes for the two endpoints, a byte for the width, 3 bytes for the color etc. So tens of bytes.

Your scan is a raster image, which is a line-by-line full storage of the scene without regard for whether any particular region is of any value to the reconstruction of the scene (i.e. blank white paper is treated the same as a line). So a simple line in a white square needs to be broken up into an arbitrary grid and each and every grid cell needs to be stored. A postage stamp at 300dpi is several hundred bytes.

For an RGB 8-bit raster image, the scene is stored as 3 b&w images called "channels" with 1 byte per pixel per channel.

I am guessing you are scanning the drawing in color at full size and choosing 300dpi, since (300x11) x (300x17) x 3 / 1024 / 1024= about 50MB.

You can reduce the size of your scan by a factor of 3 by converting to greyscale. JPeg or PNG compression may also be suitable. If the content can handle lower resolution without losing meaning (i.e. back of envelope sketch or modification to a booth/room layout etc), then try reducing the scanning dpi (try 240 or 120 etc).

If you are talking about drafting/cad style imagery, you might benefit from doing a "live trace" on the scanned image using vector-oriented software. Remember to delete the raster object after the trace or it might still be included in the file.

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Possibly helpful... What are the differences between vector graphics and raster graphics?

The "engineering firms" are most likely sending you vector PDFs. Scanning something and then saving as a Photoshop PDF creates a raster PDF. Raster PDFs are almost always larger in file size (kb).

Vector PDFs store only math used to plot the location, size, and color of paths and objects. Raster PDFs store data for each and every pixel of the PDF.

If you want sharp images with minimal PDF file sizes, you need to convert your raster scans to vector content.

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