PDF compression sounds simple, make the file smaller, but the right approach depends on what the PDF contains, how it was created, and what limit you are trying to satisfy. Get it wrong and you either end up with a file that is still too large, or one that is technically under the limit but so blurry a recruiter or caseworker cannot read it properly.
This guide covers what compression actually does, which tools are worth using, and how to choose a workflow that gives you a readable file at the right size every time.
What PDF compression actually does
A PDF is a structured container of different objects: text data, images, fonts, metadata, and structural information. Compression works on several of these simultaneously.
For text-based PDFs, those created from Word, Google Docs, or similar tools, compression mainly removes redundant font data, strips unnecessary metadata, and optimises stream encoding. This is largely lossless. The visible content is unchanged but the internal structure is more efficient. These PDFs often lose 20 to 50 percent of their size with no perceptible quality difference.
For image-heavy or scanned PDFs, the main gains come from resampling images to a lower resolution. A 300 DPI scan is overkill for a document that will be reviewed on a monitor, reducing to 150 DPI cuts image data to roughly a quarter of the original while looking perfectly clear on screen. This is lossy in the sense that fine detail is reduced, but for most document images the change is either invisible or acceptable.
The right tool for most people: target-based compression
Many PDF compressors offer abstract quality settings like "strong", "medium", or "recommended". These are frustrating because they tell you nothing about the actual output size. What you need is a tool that lets you name your target, 2MB, 5MB, 10MB, 25MB, and works backwards from there.
PDFWhisk's compressor works this way. Set a target and it runs a local optimisation pass calibrated to reach that size while preserving as much quality as possible. Because it runs in your browser, your file never leaves your device.
Choosing the right target size
Most upload limits you encounter fall into one of these bands:
- 2MB, strict job portals, some NHS and council forms, older HR onboarding systems. Text PDFs reach this easily; scanned documents may need splitting first.
- 5MB, most ATS systems, university portals, letting agent document uploads. The sweet spot for most scanned documents.
- 10MB, standard email, mortgage and tenancy packs, planning portal submissions. Achievable for almost any document without quality compromise.
- 25MB, Gmail attachments, WhatsApp, general sharing. Only the most complex design files struggle at this target.
Set a target slightly under the actual limit. If the portal says 5MB, compress to 4MB. Some systems measure in mebibytes rather than megabytes, and a small buffer prevents borderline uploads from failing.
When to delete pages before compressing
This step saves more time than most people expect. If a 20-page bank statement only needs pages 3 to 7 for a mortgage application, deleting the other pages first can reduce file size by 60 to 80 percent before any compression is applied. The compressor then needs far less aggressive settings to hit the target, which means better image quality in the output.
The same logic applies to cover pages, appendices, duplicate scans, and any pages that are blank or irrelevant to the submission.
When to split instead of compress
Some documents, particularly large scanned bundles with colour photographs, genuinely cannot reach a tight limit like 2MB without the output becoming hard to read. Forcing more aggressive compression does not solve the problem; it just creates a different one.
The better approach is to split the file into logical sections, then compress each section with a sensible target. Many portals accept multiple files, which makes this straightforward. For portals that insist on a single file, check whether they truly need everything in one document or whether they will accept a split submission via email as a fallback.
Desktop tools: what the alternatives look like
If you are on a Mac, Preview includes a Reduce File Size option via its Quartz filter. It is fast and works offline, but the quality results are inconsistent, it can produce worse results than the input for some files. Adobe Acrobat gives precise control but costs money. LibreOffice can re-export PDFs with reduced quality settings, which is useful when you still have the source document available.
For most people, a browser-based target-sized compressor is the most consistent option because the output size is predictable and it works on any device without installation.
Checking the output before you send
This is the step people skip and then regret. After downloading the compressed file, open it and look at the critical parts: small text in tables, signatures and dates, scanned reference numbers, any charts or diagrams. Zoom in to 150 percent and check that fine detail is still sharp enough to read.
If something looks questionable, step up to the next size target or use a split workflow. A readable document submitted a day later is better than an illegible one submitted on time.