Ok I did a test. This test is from a magazine, looks like 300 dpi to me. Here goes.
Scanning Notes: Scanning Printed Matter
1. Prescan: 36-24 Bit RGB, no filters. Make sure you include a step chart.
2. Use the Color (C) button and then pick highlight and shadow with eyedropper tool.
3. Set percentage to 100% @ 600dpi. For some reason, at first I could not get it to 100% unless I moved the DPI slider down to 133 but then I got it. I don't know if this was enabled or not but when I did get 100% @ 600dpi, the Output locks were unlocked (didn't really notice if they were locked or not previously).
TEST ONE
Page is from Photo Pro, looks like 300 dpi.
This scan was without descreening (17mb JPEG 12). After the scan was complete, I opened it in Photoshop, cropped (5.28 x 10.76 inches, just cropped, no recuction), rotated and spotted. Saved file(10.6mb JPEG 12). This is my working scan.
a. Then saved a PS PDF (1.6mb JPEG 5). Image looks terrific!
b. Then I went back to the working scan and saved another as a PS PDF but this time at a JPEG quality of 2. The file went down to 1,012 k and still looked terrific.
c. Then next test I did, I went back to the working scan, adjusted the levels so the white point contained 0 dot. Then converted the image to Indexed color, 128 colors, no diffusion or dither. Saved the file as a GIF (5.5 mb) It looked perfectly acceptable to me, especially for on screen viewing.
d. Opened the GIF version in Photoshop and saved as a PS PDF. Because it was indexed color, the JPEG compression option was not available so I left it at the default "Zip" and saved it. The file size actually increased to 6.6 mb.
e. Opened the working file again and converted it to a 64 color GIF. 4.4 mb.
f. Saved the 64 color GIF as a PS PDF (zip). 5.6 mb
It appears that the best results from above would be save a PS PDF at JPEG 2 from the working file which put the file just under a meg.
TEST TWO
Same image accept this time I scanned it at 300 dpi. My goal is to get the scan time down and get the best text I can at the smallest size.
The scaning time was about a quarter of the 600 dpi scan, or so it seemed. Just like the scan above, I cropped, rotated and spotted and saved as a JPEG 12. 4.8 mb.
a. Saved a PS PDF at JPEG 2. The file was 392 k and still looked good.
TEST THREE
Same image accept this time I scanned it at 150 dpi. My goal is to get the scan time down and get the best text I can at the smallest size.
The scaning time was about a minute or less. Just like the scan above, I cropped, rotated and spotted and saved as a JPEG 12. 1.3 mb.
a. Saved a PS PDF at JPEG 2. The file was 192 k but the text was not so good.
b. Saved a PS PDF at JPEG 5. 248 k and the type still looked bad but you could read it.
c. Saved a PS PDF at JPEG 5 again but this time I set the whited to 0 dot. 244 k and the type still looked bad but you could read it.
d. Saved a PS PDF at JPEG 8 with white point at 0. 384 k and the type looked soft but you could read it.
e. Saved a PS PDF at JPEG12. 1.4 mb and the type didn't look much different than example e above.
RESULTS: The 300 dpi scan that was saved as a PS PDF with JPEG 2 was only about 10 k larger than the 150 dpi scan saved at JPEG 8/12 and looked much better. The 600 dpi scan that was saved as a PS PDF JPEG 2 looked the absolute best but it was almost overkill.
The image looked perfectly acceptable on all scans, even though I didn't use any filtering. I think I got lucky.
I will try again tomorrow (if I can get the time) on newspaper articles where the line screen is lower and there is a gray paper base that I will want to get rid of.
Here is a link to the PDFs I made:
http://homepage.mac.com/megilvey/FileSharing2.html