PDF compression is not one magic operation. A scanner may have stored every page as a large photograph; a design app may have embedded oversized images; a word processor may already have produced a lean file with selectable text. Those documents need different treatment. The useful question is not “How do I get the smallest number?” but “How small can this file become while it still works for its next job?”
QuickMerge’s compressor works locally in the browser. It can re-render pages at 72, 120, or 180 DPI and rebuild the PDF, while also trying a lossless re-save. It keeps the smallest safe result and will not replace your file with a larger one. That makes it especially effective for scans and image-heavy documents, but it also means you should understand when text will be flattened into page images.
- Start with Balanced (120 DPI) for ordinary screen reading and form uploads.
- Use 72 DPI only when a strict upload limit matters more than print detail.
- Keep the original: a rasterized result can lose selectable and searchable text.
- A 0% saving can be the correct result when the source is already efficient.
First, identify what is inside the PDF
Open the document and try to select a sentence. If individual words highlight, the file has a text layer. Zoom to 300 or 400 percent: vector text should stay clean while a scan begins to show pixels. Also notice whether every page is a photograph, whether there are full-page colour backgrounds, and whether the file came directly from a phone scanner.
Scanned PDFs usually offer the largest reduction because each page is a bitmap. Born-digital reports often contain compact text instructions and may already be close to optimal. Compressing the latter by turning pages into JPEGs can make text less crisp and sometimes make the file larger. That is why QuickMerge compares the output with the source instead of assuming every conversion is an improvement.
What 72, 120, and 180 DPI mean
| Setting | Best for | Trade-off |
|---|---|---|
| 72 DPI — High compression | Email, portals, receipts viewed on screen | Small type and signatures may soften |
| 120 DPI — Balanced | Most forms, reports, and screen reading | Moderate file size with readable detail |
| 180 DPI — Low compression | Printing, diagrams, fine handwriting | Larger output but stronger detail |
DPI describes the number of rendered pixels across one printed inch. It is not a general quality score. A clean text page can look acceptable at 120 DPI on a phone, while a faint pencil signature may need 180 DPI. Always inspect the most demanding page, not only the cover.
A reliable compression workflow
- Make a copy of the original and note the upload or email limit you must meet.
- Use the PDF compressor at Balanced first.
- Open the downloaded file and inspect fine print, signatures, barcodes, charts, and page edges.
- Try High compression only if the file is still above the limit. If the document is extremely large, use Split PDF first so the browser handles smaller sections.
- Confirm page count, orientation, and filename before sending it.
This two-pass method is faster than repeatedly choosing the strongest setting. It also gives you a reference point: if 120 DPI already meets the target, there is no benefit in throwing away more detail.
Why searchable text can disappear
When rasterization creates the smallest result, each page becomes one flattened image. The words still look like words, but they are no longer text objects: copying, searching, screen-reader navigation, and text extraction may stop working. If accessibility or later editing matters, keep the original or use the lossless result. For scanned material that needs searchable text, run OCR as part of the workflow and keep both an archival source and a delivery copy.
When compression is the wrong fix
Compression will not repair a damaged PDF, improve an unreadable scan, or preserve complex interactive forms after flattening. If only a few pages are needed, splitting is usually cleaner. If the file contains duplicated pages, remove them first. If images were exported at excessive resolution, going back to the source document and exporting with sensible settings can preserve text better than post-processing.
And if the result reports no saving, believe it. A short text PDF can already be smaller than a page-by-page JPEG version. The honest outcome is to keep the original rather than manufacture a “compressed” file that is larger or blurrier.
Final checklist
- The output is below the required size limit.
- Fine print, signatures, and barcodes remain readable.
- Page count and page dimensions match the source.
- The original is stored separately.
- Searchability was checked if the document needs it.
Written and reviewed by Mehran
Founder of QuickMerge. Mehran writes practical guides around the real behavior and limits of the tools. Connect on LinkedIn.