Sending PDFs on WhatsApp, Telegram, Slack and iMessage

Email has one failure mode for an oversized file: it refuses to send, and tells you. Messaging apps have a worse one. Your PDF goes through, it arrives, it opens — and nobody notices until someone tries to read the small print and finds it unreadable.

That is the real risk with messaging. It is rarely rejection. It is silent recompression.

The limits, at a glance

Note what these numbers imply: for most messaging apps, size is not your problem. A 20 MB PDF is comfortably inside every limit except Discord's free tier. What destroys your document is the app deciding to "help".

The toggle that matters more than the limit

In WhatsApp, attaching a photo through the gallery or camera treats it as media: the app resizes it to roughly 1600 pixels on the long edge, re-encodes it as JPEG, and sends that. A scanned contract at 300 DPI becomes a soft, slightly mushy image.

Attaching the same file through the paperclip as a Document sends the original bytes, untouched, up to 2 GB.

Telegram works the same way. Send as a photo and it compresses. Send as a file and it does not.

This is also why a PDF is usually the right container. Sending a multi-page document as a single PDF file, through the document/file path, avoids the per-image recompression that happens when someone sends ten separate photos of ten pages.

WhatsApp specifics

• Documents up to 2 GB on personal and business apps, raised from 100 MB in 2022.
• Media is always recompressed unless sent as a document. HD mode, introduced later, applies a lighter pass — but it is still compression.
• The Business API is much stricter than the phone app: documents cap at 100 MB, images at 5 MB, video and audio at 16 MB. If a business messages you through an automated integration, those are the limits in play.
• WhatsApp suggests Wi-Fi above roughly 100 MB, and uploads run at your upstream bandwidth, which is usually far slower than download.

Telegram specifics

• 2 GB per file free, 4 GB with Premium — among the most generous of any messaging platform.
• Send through the file option, not the photo option, to avoid compression.
• Cloud-based, so you can send a file to yourself as a cross-device transfer.

Slack specifics

• 1 GB per file, which effectively means size is never the issue.
• On free workspaces the real constraint is total workspace storage rather than any per-file cap. If you are repeatedly hitting limits on Slack's free plan, it is the cumulative total, not the individual PDF.
• Slack does not recompress uploads, so what you send is what people get.

iMessage specifics

Apple does not publish a hard limit. Around 100 MB is the widely reported practical ceiling, and Apple's own support materials describe attachments as limited by what your carrier allows when messages fall back to MMS.

That fallback is the trap. If iMessage is unavailable — no data connection, or a non-Apple recipient — the message degrades to MMS, which carriers typically cap at around 1 MB. A document that would have gone through fine instead fails or arrives badly degraded.

When size actually does bite

There are three situations where the limit itself is your problem rather than compression:

1. Discord's free tier, at roughly 10 MB, which most large PDFs will exceed.
2. Messenger, at 25 MB.
3. Cellular uploads in general. A 200 MB PDF is technically within WhatsApp's 2 GB limit, but on mobile data it will crawl, and the recipient pays the same cost to download it.

In all three, the useful question is not "how do I squeeze it under?" but "does this need to be this big?"

Getting a PDF small enough to send comfortably

Even when the limit is 2 GB, a smaller file is a better file: faster to send, faster to download, and kinder to someone on a phone with a weak connection.

1. Check the size first. Do not compress blind — find out whether you are dealing with a 3 MB file or a 300 MB one.
2. Compress in your browser. Open the compress tool and reduce it. Processing happens locally on your device, so contracts, statements, and identity documents are not uploaded to a server.
3. Go moderate, not maximum. Over-compression is what produces the blurry scans people complain about. Stop as soon as the file is comfortable.
4. Open the result on a phone, not just on your laptop — that is where the recipient will read it.
5. Send it as a document, never as media.

If the PDF is a scan, most of the bulk is wasted resolution. Rescanning at 200 DPI in grayscale, or photographing in even light rather than a dim room, will beat any amount of post-processing.

On privacy

Messaging apps are convenient and, in the case of WhatsApp, Telegram, and Signal, end-to-end encrypted. But "encrypted in transit" is not the same as "never left your device".

For genuinely sensitive documents — passports, bank statements, medical records — reducing the file locally in your browser before it ever enters a chat is the safer default, and it costs you nothing. See browser-local vs server-based PDF tools for the concrete difference.

Before you send

• ☐Sent as a document or file, not as a photo
• ☐The recipient's app is not Discord free tier or Messenger if the file is large
• ☐Opened the received copy on a phone and confirmed the small text is readable
• ☐Using Wi-Fi if the file is over ~100 MB
• ☐Not a signed or certified document that must not be altered

Frequently asked questions

What is WhatsApp's file size limit?

Documents can be up to 2 GB on the consumer WhatsApp apps. Photos and videos sent as media are recompressed automatically, which is a quality problem rather than a size problem.

How do I stop WhatsApp from compressing my file?

Send it as a document rather than as media. Use the paperclip and choose Document instead of Photo & Video, then select the file. WhatsApp does not recompress documents.

What is Telegram's file size limit?

2 GB per file on the free tier and 4 GB with Telegram Premium. Send files through the file option rather than the photo option to avoid compression.

Is there a hard size limit on iMessage?

Apple does not publish one. Around 100 MB is the widely reported practical ceiling. If iMessage is unavailable the message falls back to MMS, which carriers cap far lower — often around 1 MB.