What does this compression setting change?
It removes unused PDF objects and compresses streams while keeping the visible page appearance. It does not downsample the photographs inside the PDF.
PDF TOOLS
Compress PDF Files
Optimize a PDF by removing unused data and compressing streams without changing its visible pages. Some PDFs may not get smaller.
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Why can an image-heavy PDF stay large?
Scanned pages and photos often account for most of a file's size. Since their resolution is preserved, this optimizer may save little or the output may not be smaller.
How can I make a scanned PDF smaller?
If the source is photos you are combining, use the Image to PDF tool's Small or Balanced image setting before creating the PDF. That compresses images at creation time; check that fine print remains readable.