How to reduce PDF file size (and why a PDF is big in the first place)
A PDF that is too large is almost always a PDF full of pictures: a phone-scanned contract, a brochure exported from a design tool, a report with screenshots on every page. The text itself weighs almost nothing. This guide explains where the bytes go, how the three compression levels in Toolkitta’s Compress PDF map to real pixels, how to choose one for a size limit, and what to do when the usual method leaves a scan unchanged. Everything runs in your browser; the file is not uploaded.
Where the bytes go
A PDF is a container of objects. Text is stored as characters plus the fonts needed to draw them; a whole book of text fits in a couple of megabytes. Vector drawings — logos, charts, table rules — are stored as drawing instructions and are tiny. Pictures are stored as embedded images, usually JPEG or losslessly compressed pixels, and they are where almost all the size lives. One 300 dpi A4 scan is 2480 × 3508 pixels; stored as a high-quality JPEG that is 1–3 MB per page, and a twenty-page scan lands at 30–50 MB.
Two other things add up quietly. Files that were edited and re-saved several times often carry the old versions of replaced objects inside them, because PDF allows changes to be appended rather than rewritten. And some producers embed full-resolution page thumbnails alongside every page. Neither is visible, and both can be dropped without changing what the document shows.
What compression actually does
Every mainstream PDF compressor works the same way: find the embedded pictures, throw away pixels the target use does not need, and re-encode what is left as JPEG at a lower quality. Text, fonts and vectors are copied through unchanged, which is why a text-only PDF barely shrinks — there is nothing to compress. Toolkitta’s Keep text method does exactly this, on your device, and additionally rewrites the file structure so that leftover objects and page thumbnails are dropped.
The word “lossy” matters. A JPEG re-encoded at a lower quality does not get its detail back, and a downscaled image cannot be upscaled to its original sharpness. That is fine for a document that will be read on a screen, and it is why the original file should be kept: compress a copy for sending, and archive the original.
Choosing a level
The three levels differ in two numbers: the resolution cap for pictures, expressed as dots per inch across the largest page, and the JPEG quality used for re-encoding. The cap is a ceiling — pictures that already have fewer pixels are never enlarged — and any picture whose re-encoded version would not be smaller is left exactly as it was.
- Light: 300 dpi, quality 85. For documents that will be printed or zoomed into: portfolios, technical drawings with photographs, anything with fine print. Savings are modest on files that were already print-ready.
- Balanced: 150 dpi, quality 75. Sharp on laptops and phones, readable when printed. The default, and the right choice for email attachments, application portals and shared drives.
- Strong: 100 dpi, quality 60. For hard limits such as “under 2 MB”. Text in scans stays readable; fine texture in photographs softens visibly.
If a portal states a limit, start with Balanced and look at the result row: it shows the exact bytes before and after. If that is still over the limit, try Strong. If Strong is not enough, the document probably has more pages than the limit allows for at any quality — split it, or ask whether the recipient can accept two files.
A worked example
Take a ten-page contract scanned on a phone at 300 dpi and saved as a 28 MB PDF. Each page is a 2480 × 3508 pixel JPEG. At Balanced the cap for an A4 page is about 1754 pixels on the long side, so each page becomes roughly 1240 × 1754 pixels — a quarter of the pixels — and quality 75 stores that in a few hundred kilobytes. The whole document lands in the 3–5 MB range, under the 10 MB limit most email providers apply, and the signatures and small print are still legible on a phone.
Now take a 40-page report written in a word processor with a logo on each page and three charts. It is 900 KB. The logo is small, the charts are vectors, and there is nothing to downscale, so Balanced returns the original or shaves only the structural leftovers. That is the correct outcome: the tool reports “Original kept” rather than handing back a rewritten file that is no smaller.
When a scan will not shrink
Scanners and some PDF producers store pages in encodings a browser cannot decode directly: JPEG 2000, CCITT fax for black-and-white pages, JBIG2, or CMYK and indexed colour for print-oriented files. The Keep text method leaves those pictures unchanged and says how many in the result row. For those files, switch the method to Flatten pages. Every page is rendered at the level’s resolution and stored as a single JPEG picture, which works for any PDF at all.
Flattening has a real cost: text is no longer selectable or searchable, links and form fields disappear, and the page is only as sharp as the chosen resolution. A scan rarely has selectable text to lose, so flattening a scan is a fair trade. Flattening a report or a form is not; use Keep text for those and accept that they may already be as small as they can be.
What is not changed, and what to check
- Open the compressed file and page through it. Look at any photograph and at small print in scans; if either is too soft, go back a level.
- Check that the page count matches, and that forms, bookmarks and links still work if the document has them — Keep text preserves them, Flatten pages does not.
- Remember that compression is not sanitization. Document metadata, hidden annotations and the text under a black box drawn over it are still in the file. Use a proper redaction tool for confidential content.
- Keep the original. The compressed file is for sending; the original is for the archive.
Encrypted PDFs, XFA forms and digitally signed documents are rejected up front with an explanation, because rewriting them would break the encryption or the signature. Export an unencrypted, unsigned copy from your PDF application first, and keep the signed original where the signature still counts.