Lossless vs Strong PDF Compression – Which One Do You Need?

Use Lossless compression for PDFs created on a computer (exported from Word, Google Docs or design software) when you need text to stay selectable, searchable and sharp. Use Strong compression for scanned documents and photo-heavy PDFs where file size matters more than selectable text: it redraws each page as a JPG image at the resolution and quality you choose, which can cut scans down dramatically. If you're unsure, try Lossless first; it can't hurt the document.

Here's how to decide, with the numbers behind it.

Where does a PDF's size actually come from?

A PDF is a container. Text is stored as characters plus font data, which is compact. Vector graphics (charts, lines, logos) are stored as drawing instructions, also compact. Images are stored as image data, and that's where megabytes come from. A 40-page text report might be a few hundred kilobytes; a 10-page scan, where every page is an image, can easily be many megabytes.

That's why the two modes behave so differently.

What does Lossless mode do?

It rewrites the PDF's internal structure more efficiently, packing many small internal objects into compressed "object streams." Nothing you can see changes: text, fonts, links, images and page sizes are all kept as they are.

The savings depend on how the file was made. PDFs from some office tools have a loose structure and shrink noticeably; PDFs that were already optimized barely change. Lossless mode doesn't recompress the images inside, so it won't do much for a scan.

What does Strong mode do?

It renders every page to a picture at the resolution you choose, saves it as a JPG at the quality you choose, and builds a new PDF from those images at the original page size. The output looks like the original but is now made of images.

The trade-off is significant:

• Text is no longer selectable or searchable. You can't copy from it, and search inside it won't work.
• Links and form fields stop working, since they're no longer part of the page.
• Vector sharpness becomes resolution-dependent. Zoom in far enough and you'll see pixels.

For a scan, none of that matters: it was already images. For a digital report, it usually matters a lot.

How do DPI and quality affect the result?

Resolution sets how many pixels each page gets. For a US Letter page (8.5 × 11 inches):

Resolution	Page in pixels	Pixels vs. 110 DPI	Good for
72 DPI	612 × 792	about 0.4×	Smallest file, quick previews
110 DPI (default)	935 × 1210	1×	Reading on screen
150 DPI	1275 × 1650	about 1.9×	Comfortable zooming, small print
200 DPI	1700 × 2200	about 3.3×	Printing

Image quality (20–90%, default 60%) controls JPG compression on top of that. For text scans, 50–70% is usually clean; drop lower and letters get fuzzy halos.

Rule of thumb: lower the DPI before you lower the quality. Fewer pixels at decent quality look better than many pixels at poor quality.

Decision table

Your PDF	Mode	Suggested settings
Exported from a word processor, mostly text	Lossless	—
Contract you or others need to search or copy from	Lossless	—
Phone or scanner scan of paper documents	Strong	110–150 DPI, 60%
Scan that will be printed again	Strong	200 DPI, 70%
Brochure with big photos, text must stay selectable	Lossless, or rebuild it with smaller images	—
Fillable form	Lossless	Strong would flatten the fields

How to compress a PDF step by step

1. Open Compress PDF and choose a file. Its current size is shown next to the name.
2. Under Compression, choose Lossless — keeps text selectable or Strong — converts pages to images (best for scans).
3. For Strong, pick a Resolution (72, 110, 150 or 200 DPI) and set Image quality.
4. Click Compress PDF. In Strong mode, a progress bar shows pages being rendered.
5. Compare the original and new sizes in the result, then download.

If the result wouldn't be smaller, for example a text-only PDF in Strong mode, where crisp vector text is far more compact than a picture of it, the tool keeps your original and says so. The PDF is processed entirely in your browser.

Tips

• Keep the original. Especially after Strong mode, since you can't get the text layer back.
• Compress parts, then merge. In a bundle with both a typed letter and scans, compress only the scans in Strong mode, then combine everything with Merge PDF. The letter stays searchable.
• Shrink photos before they go into a PDF. If you're building a PDF from photos, run them through the Image Compressor first.

FAQ

Why is the Strong result still large?

Probably a high DPI on many pages. Each step up multiplies the pixel count; try 110 DPI before lowering quality.

Can I make a scanned PDF searchable again after Strong mode?

Not with this tool. Searchable scans need OCR (optical character recognition), which recognizes letters in images. Run OCR on the original, not the compressed copy.

Does Lossless mode change how the PDF prints?

No. It reorganizes how the file is stored, not what's on the pages.

Pick your mode from the table and try it in Compress PDF.