A PDF that should be a few hundred kilobytes turns out to be 15 MB, and the upload form refuses it. The fix depends on what is inside the file, so it is worth a minute to understand where the size comes from before you start squeezing.
Where the size comes from
Text is cheap. A hundred pages of plain text with embedded fonts is often under 1 MB. What makes PDFs heavy is almost always images: scanned pages, phone photos of documents, and pictures pasted into a Word file before it was saved as PDF.
A phone camera captures a page at 12 megapixels or more. Saved inside a PDF without downsampling, one page can be 3–5 MB. Ten pages and you are past most upload limits. Other causes are smaller: fonts embedded in full instead of as subsets, duplicate images repeated on every page (logos, letterheads), and leftover data such as thumbnails and editing history.
Lossless compression: always safe
Lossless compression rewrites the PDF more efficiently without changing how anything looks. It removes unused objects, compresses streams that were stored raw and merges duplicates. Text stays selectable and images keep every pixel.
Expect savings of 5–30% on documents exported from Office, and very little on scans. Try it first: if the result is small enough, you are done and nothing has been lost.
Image compression: where the big savings are
Strong compression re-encodes each page’s images at a lower resolution and JPEG quality. For a scan, 150 dpi is perfectly readable on screen and in print at normal size; 100 dpi is still legible for typed text. Going from a 300 dpi colour scan to 150 dpi greyscale typically cuts the size by 80–90%.
- Typed documents and forms: 150 dpi greyscale is clear and small.
- Documents with photos or stamps in colour: keep colour, use 150 dpi.
- Signatures and handwriting: check the result at 100% zoom; very low quality smudges thin pen lines.
Some strong modes re-draw each page as a picture. The page looks the same, but its text is no longer selectable or searchable. If that matters, run OCR afterwards.
Hitting an exact limit like 100 KB
Government and exam portals often cap PDFs at 100 KB, 200 KB or 300 KB. A rough budget: a single A4 page of typed text scanned at 150 dpi greyscale is 60–150 KB. So a one-page certificate fits in 100 KB, but a three-page form usually needs 200–300 KB or lower resolution.
Order matters. Remove pages you don’t need first (blank backs of scans, instructions pages), then crop wide margins, then compress. Each step makes the next one go further. Our 100 KB tool tries progressively stronger settings until the file fits, and tells you if it cannot get there without becoming unreadable.
Prevent big PDFs in the first place
- Scan documents at 150–200 dpi, not 600 dpi, unless you need to print enlargements.
- Use the scanner’s “document” or “greyscale” mode for text.
- When saving from Word, choose “Minimum size (publishing online)” in the PDF options.
- Phone scanner apps usually have a size or quality setting; medium is enough for forms.
Is it safe to compress a PDF online?
Most PDF compressors upload your file to their servers. For ID cards, bank statements and certificates that is a real risk. Myriadfile’s PDF tools run inside your browser, so the document never leaves your device. You can check this yourself: open the page, disconnect from the internet, and compress a file.
Sources: Adobe Acrobat help, reducing PDF file size; ISO 32000-2 (PDF 2.0).