How to Reduce PDF File Size Without Losing Quality
Most of the size in a bloated PDF comes from images, not text. A ten-page contract with no images might be 200KB, while a ten-page scanned document with full-resolution photos can easily hit 40MB. Understanding that difference is the key to compressing smartly instead of blindly.
What compression actually changes
PDF compression tools typically do two things: downsample embedded images to a lower resolution, and increase JPEG compression on those images. Text and vector graphics are usually left alone, since they take up very little space to begin with and compressing them wouldn't meaningfully reduce file size anyway.
Choosing a compression level
A light compression setting is usually enough for documents that will only be read on screen, since most monitors can't display more detail than a 150 DPI image provides anyway. A medium setting is a safe default for general sharing. A high compression setting is best reserved for documents you just need to be small and don't need to print, since it will visibly soften photos and scanned text.
Compressing scanned documents specifically
Scanned pages are the worst offenders because scanners often save at 300 or 600 DPI by default, far higher than needed for reading on a screen. If your original scan is unnecessarily high-resolution, you can often cut file size by 80% or more with almost no visible difference in a compression tool built to handle scans specifically.
What not to do
Avoid repeatedly compressing the same file through multiple tools or multiple passes, since each pass re-compresses already-lossy images and the quality loss compounds every time. Compress once, check the result, and if it's still too large, go back to your original uncompressed file and try a stronger setting from scratch rather than compressing the already-compressed version again.
A practical target
Most email providers cap attachments around 25MB, and many web forms cap uploads even lower. If you're aiming to get under a specific limit, it helps to know your target size before you start rather than compressing repeatedly by trial and error.
