How to compress a PDF for email

Why is my PDF so large? Where the megabytes come from

A page of plain text is tiny. The words are stored as characters plus a reference to a font, so a 40-page contract with no pictures usually weighs a few hundred kilobytes at most. When a PDF runs to 30, 60 or 200 MB, the text is almost never the reason. Three things usually are.

• Images at scanner resolution. A photo dropped into a report straight from a camera can be 4000 × 3000 pixels, even if it is printed 8 cm wide on the page. The PDF keeps every one of those pixels. Multiply by twenty photos and you have a file that is mostly invisible detail.
• Pages scanned as pictures. A scanned document is not text: each page is a photograph of a page. At 300 dpi an A4 page is roughly 2480 × 3508 pixels, and if the scanner saved it with light compression or as a lossless image, each page can weigh a megabyte or more. A 50-page scan quickly passes the size an email will accept.
• Fonts embedded more than once. Some programs embed a full copy of the same font every time you merge or export, or embed the whole typeface instead of the characters actually used. It rarely adds up to tens of megabytes, but in files assembled from many sources it can be a few.

There is also some structural fat: duplicate objects left behind after editing, metadata, thumbnails, old revisions saved incrementally. It is worth removing, but it is the smaller part of the problem.

The practical consequence: before you compress anything, ask what the PDF contains. A file of scans or photos can shrink a lot. A text-only PDF that is already small will barely move, and no tool can change that, because there is nothing heavy to remove.

How to reduce PDF size without losing quality: what to touch and what to leave

Reducing a PDF's size without losing quality really means removing what nobody will ever see. That depends on where the document is going.

What you can shrink safely if the PDF will be read on screen:

• Image resolution down to 150-200 ppi. A laptop screen showing a full A4 page displays it at well under 150 pixels per inch. Images above that are detail that only appears if someone zooms in to 300 % or more. For reports, invoices, CVs and anything read on a monitor or a phone, 150-200 ppi is plenty.
• Recompress photos as JPEG at high quality. Photographs stored losslessly are many times larger than the same photo as a good JPEG, and at high quality the difference is not visible at normal reading size.
• Remove duplicates and leftovers. Repeated images, unused objects and metadata can go with no visual effect at all.

What you should not touch:

• Do not go below 150 ppi on a document that will be printed. Print needs far more density than a screen. A brochure, a poster proof or a technical drawing that a print shop will use should stay at its original resolution (usually 300 ppi). Shrinking it to email it is fine only if you also keep the original for the printer.
• Line art and small text inside images. Plans, diagrams, screenshots with small type and signatures are the first to suffer: aggressive JPEG compression leaves visible artefacts around sharp edges. If the PDF is full of them, use the lighter setting.

A related point for anyone preparing images before they go into a document: it is much cleaner to size them correctly first. If you are building a report or a catalogue, you can compress images to the size they will actually be shown at before you place them, and the PDF never gets heavy in the first place. Our guide on how to compress images without losing quality explains the same trade-off for JPEG and WebP.

PDF too large to attach? Email attachment limits explained

If you get a message saying your PDF is too large to attach, you have hit your email provider's limit. The common free email services set attachment limits somewhere around 20-25 MB per message, but each provider publishes its own figure, and it can differ for sending and receiving, for free and business accounts, and for the total of all attachments together. Check your provider's help page rather than trusting a number from memory.

There are three reasons to aim well below that limit rather than just under it.

• Attachments grow in transit. Email encodes binary files as text, which adds roughly a third to their size. A 20 MB PDF can travel as about 27 MB, so a file that looks legal on your disk can still be rejected.
• The recipient's limit matters too. Your provider may accept 25 MB, but a company mail server on the other side may cap incoming messages at 10 MB, and you will often not get a clear bounce.
• Portals are stricter than email. Job applications, tax and visa forms, university admissions and insurance claims often accept 2, 5 or 10 MB per file. If you are compressing a PDF for one of those, the portal's own limit is the target.

As a rule of thumb, aim for under 10 MB for email to someone outside your organisation, and for whatever the portal states if you are uploading a form. If the document cannot get there without becoming unreadable, splitting it is the honest answer, and we cover that in the last section.

Compress a PDF for free on your computer, without uploading it

Most online compressors ask you to upload the document to a server. For a menu or a flyer that is fine. For a signed contract, a payslip, a medical report or a passport scan, it means handing a copy of a sensitive file to a company you know nothing about, just to make it smaller.

Airpic PDF is our desktop app for Mac and Windows, and it does the work locally:

• Nothing is uploaded. The PDF is opened and compressed on your own computer. It never reaches our servers or anyone else's.
• No size limit. Because there is no upload, a 400 MB scan is just a larger file for your computer to process, not a rejected upload.
• Free, with no account. You download the app, open the PDF and compress it.

One honest note before you install it: the installers are not yet signed with an Apple or Microsoft developer certificate, so macOS and Windows will show a security warning the first time you open it. The Airpic PDF page explains how to open it anyway and how to check the file's checksum. If you are on a work computer where you cannot install software, the same page also lets you compress a PDF in the browser: it runs as JavaScript on your device, so the file is not uploaded either.

How to check the result is still good. A smaller number on its own proves nothing. Open the compressed PDF and the original side by side, go to a page that contains a photo or a scanned area, and set the zoom to 100 %. Look at the edges of text inside the images, at faces and at fine lines. If you cannot tell them apart at 100 %, nobody reading the document on screen will. Then zoom to 200 % on a page with small print: if the scanned text is still readable there, the compression was safe.

We ran this ourselves when testing the browser version: a scanned page went from 253 KB to 87 KB, a reduction of 66 %, while a text-only PDF went from 51 KB to 47 KB. That second figure is the useful warning: if your PDF is mostly text and is still too big, compression is not the fix. Look instead for something embedded that does not belong, such as a huge logo repeated on every page, or split the file.

Step by step: compress a PDF in Airpic PDF (and split it if it is still too big)

The app's interface is in Spanish, so the button names are given here with their translation.

1. Open the PDF. Drag it onto the Airpic PDF window or use the open button. On a Mac you can also right-click the file in Finder and choose Airpic PDF under Open With. Note the current size shown for the document.
2. Click «Comprimir» (Compress). You get two levels.
3. Choose the level for the destination.
   • «ligera» (light) only optimises the structure: duplicates, unused objects and leftovers go, the images stay exactly as they are. Use it for documents headed to a printer, for plans and drawings, and for any PDF where image detail matters.
   • «fuerte» (strong) also recompresses the images and brings them down to 120 dpi. This is the right choice for scans and photo-heavy reports that will be read on screen or sent by email. It is below the 150 ppi floor for print, so do not use it on a file that is going to a print shop.
4. Read the before and after size. The app shows both. If strong compression brought a 38 MB scan under 10 MB, you are done. If it barely changed, the PDF was mostly text already (see the previous section).
5. Check a page at 100 %. Before saving, look at the page with the most detail. If it is not good enough, click «Deshacer» (Undo) to get the original back and try the light level.
6. Save it under a new name. Keep the original. A compressed copy is for sending, the original is for archiving and for printing.

If it is still too large. Some documents cannot get under an attachment limit without being damaged, for example a 300-page scanned file. In that case, split it: open the same PDF and use split a PDF to cut it by page range (pages 1-100, 101-200 and so on), then send each part in a separate email or upload them one by one. If the recipient needs to search the text of a scanned document, running OCR on the PDF first makes it searchable without adding much weight.

For everyday sending, this is the order that works: strong compression for anything read on screen, light compression for anything printed, split only when the first two are not enough. You can compress a PDF with Airpic PDF on your computer, free and without uploading it.

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