Why a PDF or image got bigger after compressing it

A file gets bigger after compressing when it was already efficient. A PDF of typed text stores letters as tiny instructions, so turning its pages into pictures adds data. A JPG that was already compressed, re-saved at a high quality or converted to PNG, also grows. Keep the original, and compress only files made of photos or scans.

Why “compress” does not always mean “smaller”

Compression is not a single operation that squeezes any file. Each tool makes a trade: it replaces what is in the file with a different way of storing it, and hopes the new way is shorter. That works when the original is wasteful — a 12-megapixel photo of a receipt, a scan saved at 600 dpi in full colour. It backfires when the original was already lean.

Text PDFs

A PDF exported from Word or Google Docs does not contain a picture of each page. It contains the words, the font to draw them with, and their positions. A page of text can be a few kilobytes. Browser-based PDF compressors, including Compress PDF on this site, get their big savings by redrawing every page as an image and re-encoding it. For a scan that is a huge win, because the page was an oversized image already. For a typed document it replaces a few kilobytes of text with a picture of that text, which is larger — and the words can no longer be selected. The tool warns you when the result is bigger than what you started with; when it does, keep the original.

Images that were already compressed

JPG compression throws away detail the eye is unlikely to miss. Once that has happened, re-saving the image cannot throw the same detail away twice, so the savings get smaller each time. Worse, if the original was saved at quality 70 and you re-save it at quality 95, the encoder faithfully preserves the blocky artefacts from the first save at high quality, and the file grows while looking no better.

Changing format in the wrong direction

PNG is lossless: it keeps every pixel exactly. That is ideal for screenshots and logos with flat colour, and wasteful for photos. Converting a 300 KB JPG photo to PNG commonly produces a file several times larger. The same happens converting WebP or AVIF to JPG — the newer formats are more efficient, so the JPG version is bigger even at a similar quality.

How to tell in advance

• For a PDF, try selecting a word. If you can highlight text, the page is stored as text, and image-based compression will probably make it bigger. If the whole page highlights as one block, or nothing highlights, it is a scan and will shrink well.
• Compare size with page count. A typed document under about 100 KB per page has little left to lose. A scan at 1–3 MB per page has plenty.
• For an image, check the format and size together. A photo that is a PNG, or a JPG of several megabytes straight off a phone, will shrink. A JPG already a few hundred kilobytes at a modest pixel size will not shrink much more without visible damage.

What to do instead

1. Typed PDF too big? Look for what is heavy inside it — usually photos or a logo placed at print resolution. Re-export from the original program with a smaller image setting, or replace the photos with smaller copies before exporting.
2. Photo too big? Reduce the pixel dimensions first. A photo going on a web form or into an email rarely needs more than 1600–2000 pixels on its long side, and halving the width and height removes about three quarters of the pixels. Compress Image can resize and compress in one pass, or aim at a target such as 200 KB directly.
3. Always start from the original. Compress the best copy you have, once, at the setting you need. Compressing an already compressed copy loses quality without saving much space.
4. Pick the format to suit the content. Photos as JPG or WebP; screenshots, diagrams and anything with transparency as PNG. The Image Converter converts in batches and shows you the output size, so you can compare before you commit.

Mistakes that make it worse

• Running a file through several compressors in a row, hoping each one helps a bit.
• Taking a screenshot of a PDF page to “make it smaller” — it usually does the opposite.
• Zipping a PDF or JPG. Both are already compressed, so a ZIP saves almost nothing.
• Choosing the maximum quality setting “to be safe”. Near 100, size climbs steeply while the visible difference is negligible.

Checklist

• Is it a scan or photo (will shrink) or typed text (will not)?
• Compressing the original, not a copy that was already compressed?
• Pixel size reduced to what the destination actually needs?
• Format matched to the content: JPG/WebP for photos, PNG for flat graphics?
• Result compared with the original before you delete anything?

For hitting a specific upload limit, see getting a PDF under 100 KB, 500 KB or 1 MB.