How to compress a PDF without uploading it
A PDF that is too big to email is one of the most common document problems. Most online compressors solve it by having you upload the file to their server. A2ZDocs Compress does the same job inside your browser, so the file never leaves your device. This guide explains where the size comes from, what compression changes, and how to choose a setting.
Why PDFs get big
A page of plain text takes up very little space. A few things usually account for most of a large PDF's size:
- Photos. Pictures straight from a phone or camera often have far more detail than a printed page or a screen can show.
- Scans. A scanned page is a picture of the whole page. Scanning in colour at high resolution makes large files, and a 30-page scan is 30 pictures.
- Embedded fonts. So that they look the same everywhere, PDFs carry the fonts they use. Usually only the characters needed are included, but a document that embeds several complete fonts carries extra weight.
- Leftovers. Files that have been edited many times can contain old content that nothing uses any more, and some programs store data without compressing it at all.
How compression works
Text and drawings are already stored efficiently, so there is rarely much to gain there. What compression can do is tidy the file structure and, above all, store pictures more efficiently. Compress has three levels:
- Lossless only tidies the file. It removes objects nothing refers to, compresses data that was stored uncompressed, and packs the file's internal structure more tightly. Nothing you can see changes.
- Balanced does the same tidying, then re-saves the pictures inside as JPEG images at moderate quality, with no more detail than a full page needs at 150 dpi. This is the default, and it suits most documents that will be read on screen or printed in an office.
- Strong goes further: pictures are limited to 100 dpi and saved at lower quality. Files usually get smaller still, but photos look softer and small print in scans can blur.
A picture is only replaced when the new version is clearly smaller. If the compressed file as a whole is no smaller than the original, A2ZDocs keeps your original and tells you it was already well compressed. After each run you see the old and new size, and how many images were re-encoded.
No level turns text or vector graphics into pictures, so text stays sharp, searchable and selectable.
Choosing a level
- Start with Balanced. For documents with photos or scans it usually gives a good saving with little visible change.
- Use Strong when you need to get under a size limit and the document is mainly for reading on screen. Check small print and signatures afterwards.
- Use Lossless when picture quality must not change, for example artwork, photos meant for printing, or scans where every detail matters.
You can compress several PDFs in one go. Your original files are never changed, so if one level does not give the result you want, try another on the original.
Fitting under email attachment limits
Email services cap the size of attachments. Gmail's limit is commonly given as around 25 MB and Outlook.com's as around 20 MB, and many workplace mail systems set their own, sometimes lower, limits. Attachments are also encoded for sending, which makes them roughly a third larger in transit, so a file just under the limit can still be refused.
- Aim comfortably below the limit, not just under it.
- Compress with Balanced first, then Strong if you need to.
- Remove pages the recipient does not need with Delete pages.
- If the file is still too big, use Split with "Files under a size limit" to cut it into parts that each fit, and send them in separate emails.
- If you are making a PDF from phone photos, Scan to PDF crops and cleans up each photo first. Compress the result if it is still large.
Why not upload sensitive files to any site you find
A search for "compress PDF" brings up many free sites, and most of them work by uploading your file to their servers. Many say they delete files after a few hours; others say little about what happens to them. Even with good intentions, an upload means a copy of your document exists, for a while, on a computer you do not control, in a country you may not know, under a privacy policy you probably have not read.
For a party invitation that hardly matters. For a payslip, a passport scan, a medical report or a client's contract, it might. With A2ZDocs the file is processed on your own device. The page's Content-Security-Policy stops it from connecting to any server other than a2zdocs.com, and the only things it fetches from there are its own code, fonts and language data. The Privacy Policy has the details.
When compression will not help
- Text-only documents. A report made of text and simple charts is usually small already, with no big pictures to shrink.
- Files that are already compressed. If the pictures were already saved at modest quality, there is little left to gain, and A2ZDocs will keep your original.
- Some kinds of image. Pictures in CMYK colour (common in print-ready files), black-and-white scans stored in special compact formats, and images with masks are left as they are. The result tells you how many images it could not re-encode.
- Fonts and detailed drawings. Compress does not remove embedded fonts, and detailed vector drawings such as maps or technical plans stay as they are.
- Documents that are simply big. A long, image-heavy report may stay large whatever you do. Splitting it, or sharing it another way, can be better than squeezing every page.
Whichever level you choose, open the compressed file before you send it and check that photos, signatures and small print are still clear.