A PDF that is fine for email is often far too large for an application portal. Scanned certificates saved by a phone app, bank statements exported at print resolution, or a document that went through a scanner at 600 DPI can each run to several megabytes, and the upload box says "maximum 200 KB". Most online PDF compressors ask you to upload the document to their server — not an attractive idea for mark sheets, identity documents or bank letters.
This compressor works differently. It opens the PDF in your browser with pdf.js, the open-source PDF engine used by Firefox (licensed Apache-2.0 and served from this website), draws each page, and writes a new, smaller PDF — all on your device. You choose the size limit; it finds the sharpest version that fits.
Why are PDFs too big for portals?
The size of a PDF depends mostly on what is inside it. A PDF made from typed text with embedded fonts can be a few kilobytes per page. A PDF made from scans is a container of photographs — one per page — and its size depends on the resolution and compression of those photographs. Scanner software often uses 300 or 600 DPI in colour with light compression, which is excellent for archiving and wasteful for a verification screen. Phone "scan to PDF" apps vary a lot; some produce compact files and others embed full-resolution camera images. Portals set tight limits because they store huge numbers of documents and officials only need to read them on screen.
How does the compressor decide the quality?
Two settings control the size of a scanned page: its resolution (how many pixels) and its JPEG quality (how much detail each pixel keeps). The compressor searches both, in the order that keeps documents most readable:
- It renders every page at 150 DPI — sharp enough for normal-sized print on screen and on paper.
- It tries JPEG quality 85 %. If the whole PDF fits, done.
- If not, it binary-searches the quality between 20 % and 85 % to find the highest value that fits.
- If even 20 % is too big at 150 DPI, it steps down the resolution — 120, 100, 85, 72, 60 and finally 50 DPI — and repeats the quality search at each step.
- The first resolution where a fitting quality exists wins, so you get the highest resolution possible, at the best quality for that resolution.
Pages are rendered at their real paper size, so an A4 page at 150 DPI becomes about 1,240 × 1,754 pixels, and very large pages are capped at 3,000 pixels on the long side. The limit counts 1 KB as 1,000 bytes — the stricter reading — so "200 KB" means at most 200,000 bytes, which passes whichever definition the portal uses.
What changes in the compressed PDF?
Because every page is redrawn as an image, the compressed PDF is an image-only PDF. That is exactly what a scanned document already is, so for scans nothing meaningful changes except size. For PDFs that contain real text, be aware that:
- Text can no longer be selected, copied or searched (it looks the same, but it is now pixels).
- Clickable links, bookmarks, form fields, attachments and digital signatures are not carried over.
- Page sizes and orientation are kept, and the page order is unchanged.
For uploading supporting documents to a portal, none of these usually matter — officials look at the pages. But don't use the compressed copy as your only copy of a digitally signed document (for example, a DigiLocker-issued certificate whose signature needs to stay verifiable). Keep the original and upload the compressed copy only where the portal's size limit forces it.
How small can a PDF get — 100 KB, 200 KB, 500 KB, 1 MB?
It depends mostly on the number of pages. As a rough guide for printed documents with ordinary-sized text:
- 1–2 pages under 100 KB: usually comfortable at 100–150 DPI.
- 3–5 pages under 200 KB: usually readable; small print may need Black & white.
- 10 pages under 500 KB: about 50 KB per page, fine for most text.
- 20+ pages under 1 MB: possible, at lower resolutions; check fine print carefully.
Photographs and colourful certificates with backgrounds, seals and patterns need more bytes than plain black text on white. Documents photographed in poor light carry noise that costs bytes, so a clean re-scan can beat any amount of compression.
When should I use Black & white?
The Black & white option converts each page to greyscale before compression. For typed or printed documents, it typically makes the file noticeably smaller at the same resolution — which means the compressor can keep a higher DPI under your limit and the text stays sharper. Leave it off for documents where colour carries meaning: coloured stamps and seals that officials expect to see, photographs, colour-coded mark sheets, or when the portal asks for colour scans.
What if it can't reach my limit?
If the PDF doesn't fit even at 50 DPI, the tool tells you instead of producing an unreadable file. Options, in order of preference:
- Try Black & white if colour isn't required.
- Remove unnecessary pages — blank backs of certificates, instruction pages, duplicate copies. Rebuild the PDF from just the pages you need with image to PDF.
- Split the document if the portal allows more than one upload (for example, one PDF per certificate).
- Re-scan with better light and tighter cropping, which reduces bytes at the source.
- Check the requirement — sometimes the limit is per page, or MB rather than KB.
How long does compression take?
Everything runs on your own processor, so speed depends on your device and on the number of pages. In our automated test on a laptop-class machine, a 6-page, 1.27 MB PDF made of text and large photographs was compressed to 193 KB (limit 200 KB) in about 3–4 seconds, and to 96 KB in Black & white (limit 100 KB) in about 3 seconds. For that heavy, photo-filled test file the compressor had to go down to 72 DPI and 50 DPI respectively; ordinary text documents usually stay at a higher resolution for the same limit because they compress far better.
Longer documents take proportionally longer, because every page is drawn and encoded several times during the quality search. Phones are slower than laptops, and very long PDFs (dozens of pages at a tight limit) can take a minute or more and use a lot of memory — if a phone struggles, try a laptop, or split the document first. The first run also downloads the pdf.js engine (about 1.8 MB, cached by your browser afterwards, so later runs start immediately). The status line always shows which page and resolution are being processed, so you can see it is making progress.
What about password-protected PDFs?
PDFs that need a password to open (for example, some bank statements and e-Aadhaar downloads) can't be read without it, and the tool says so. Open the PDF in your PDF viewer with the password and use "Print → Save as PDF" to make an unprotected copy, then compress that. Portals generally reject password-protected uploads anyway, so this step is needed regardless.
Is my PDF uploaded anywhere?
No. The pdf.js engine files download from this website to your browser; your document never goes the other way. Rendering, compression and writing the new PDF all happen in memory on your device, and the result is handed to your browser's download. You can confirm this in the developer tools' Network tab — there is no request carrying your file. Once the page and the engine have loaded, it keeps working offline.
Should I compress the PDF or rebuild it from images?
If you still have the original page images (phone photos or scans), building the PDF with image to PDF and its own size limit gives you more control: you can crop, rotate and reorder pages first. If all you have is the PDF — or it came from someone else — compress it here. For single photos and signatures rather than documents, use compress an image to a KB target instead; portals nearly always want those as JPG, not PDF.
Size limits and accepted formats are set by each portal and change between notifications. Verify with the official notification before you upload.