How to compress a PDF without losing quality

Step 1: find out what kind of PDF you have

Open the PDF and try to select a word with your finger or mouse. If the text highlights, it is a text PDF: fonts and vector drawings, usually small to begin with. If nothing selects and the page behaves like one picture, it is a scan or a photo PDF, and the images are what take the space.

Text PDFs exported from Word or Google Docs are typically 50 KB to 500 KB. Scanned PDFs from a phone app are 1 to 5 MB per page.

Step 2: lossless first, if the text must stay searchable

1. Open Compress PDF and drop the file.
2. Choose Light. This rewrites the file structure, removes duplicate objects and unused data, and keeps every font and the text layer.
3. Download and check the size. Savings are 5 to 30 percent on text PDFs and almost nothing on scans, because the pictures are untouched.

Light is the only mode that keeps the text selectable and searchable. Use it for contracts, CVs you will send to recruiters, and anything a recipient might need to copy from.

Step 3: re-render scans at a sensible resolution

For scans and photo PDFs the real lever is resolution. A phone scanner app saves pages at 300 dpi or more, which is print quality. For reading on a screen 120 to 144 dpi is indistinguishable, and that alone cuts a 4 MB page to about 300 KB.

Choose Balanced for 144 dpi, which looks like the original on any screen, or Strong for 96 dpi when the limit is tight. Both turn pages into images, so text stops being selectable; the appearance is unchanged.

Step 4: hit a hard limit with Target size

1. Choose Target size and type the limit the portal gives, for example 500 KB.
2. Press Compress PDF. The tool renders the pages at 144 dpi, checks the size, and steps down through 120, 100, 96, 80 and 72 dpi, switching to greyscale at the end, until the file fits.
3. If the limit is not realistic for the page count, the tool shows the smallest file it reached and says so. Split the document and compress the part the portal actually needs.

The dedicated pages for 500 KB and 1 MB open with the limit already set.

How to keep quality high

Scan at 150 dpi instead of 300 when you still have the paper. Compress each document separately rather than one merged file. Remove blank pages first. Keep the original for printing and use the compressed copy for uploads and e-mail. And never compress a compressed copy a second time; start from the original each time.