Photoshop :: Can't Save Adjustments

Question: Why won't layer adjustments save?

Details:

I work for a music publisher that wants to make older publications available for download via their website.

I scan the sheet music and get a PDF as a result.

Some pages are too light; some need to be rotated a bit.

I open the PDF and export each page as a TIFF.

Then I open the TIFFs in Photoshop.

Rotating the image is no problem.

A Levels Adjustment Layer gives me darker blacks, higher contrast.

I save the TIFFs, then use Acrobat Professional 8.x to create a PDF from multiple files.

I combine the TIFFs, save the new file.

The rotation changes are there in the new PDF, but the level changes aren't.

If I open the TIFFs again, they look fine -- meaning that the Adjustment Layer is still present.

Next, I tried merging layers (there are just two: Background and the Adjustment Layer) and saving the TIFF again.

Close the file; reopen it in Photoshop.

One layer (Background) which seems to have no change from the pre-adjusted original.

I've tried saving in different formats (Photoshop PDF, JPEG, EPS, PSD) before and after merging or flattening.

The only thing that seems to work is the Photoshop PDF format. When I open that in Acrobat, the level adjustments are preserved.

But the resulting PDF is huge. To illustrate:

A 12-page folio typically contains only simple lineart (no photos) and text on page 1 and music (just a font, not an image) on pages 2-12. Sometimes there is an EPS barcode on the final page.

The PDF my scanner produces usually weighs in at 3MB.

The recombined PDF made of 12 Photoshop PDFs (TIFFs extracted from the original PDF, then adjusted in Photoshop and resaved) is typically 60+MB. Using Acrobat's PDF Optimizer gets the size down to 20MB or so.

But this is huge. These files should be around 1-2MB tops.

I have hundreds of pieces to scan, adjust, and prep for downloading.

Finally, my specs:

iMac
2 GHz Intel Core 2 Duo
3 GB RAM

Mac OS X 10.5.7

Photoshop CS3 Extended 10.0.1