How to compress a PDF for email without ruining it

The numbers on the three buttons

I measured one file three times so I would stop talking about compression as a mood. Source: 28-page proposal, 22.4 MB, six full-bleed photos and a lot of real text. Desktop Chrome. Same machine, same hour.

All three failed the only test that matters if someone might copy a sentence later. Compress PDF does not rewrite image streams inside the old PDF. It opens the file with PDF.js, draws each page to a canvas, and asks jsPDF for a new document made of those JPEGs. High, balanced, and small are the three configs in the page script: {scale:1.7,jpeg:.88}, {scale:1.35,jpeg:.72}, {scale:1.05,jpeg:.58}. There is no fourth mode that “keeps text and just squeezes photos.” If a vendor promises that, they are describing a different algorithm.

I still use it every week. I just stopped using it on the only copy.

Why the email bounced at 18 MB

The gateway said 20 MB. The attachment was 18.2 MB and it still bounced. Base64 encoding, which is what a lot of mail systems do to binary parts, adds about a third. 18.2 × 1.37 is over 24. The honest target for a “20 MB mailbox” is closer to 14 MB of PDF bytes. I aim under 12 when I do not control the recipient’s server.

Balanced got me to 4.6 MB. That cleared the gateway with room for the signature image in the mail itself. High would also have cleared, and the photos looked better. I sent balanced because two of the photos were already soft phone shots. The extra scale was not buying detail the camera had never captured. On a different proposal, one with a vector site plan, I sent high and kept the master uncompressed. The site plan’s thin lines turned gray at the small setting. At high they were still stubby, but a reviewer could see the setback dimension.

What I look at before I trust the small file

Zoom to 200% on three places: a face or product photo, a chart with thin gridlines, and the smallest body text. Photos usually survive balanced. Charts and 8-point footnotes are where JPEG blocks show up. If the footnote is a number someone will type into a spreadsheet, do not make them read it off a muddy JPEG. Split that page out of the master as real PDF, or type the figure in the email and attach the compressed deck for context.

Text-only PDFs disappoint people who expect magic shrinkage. A 400 KB policy with embedded fonts has little redundant image data. The compressor will still rasterize it and can hand you a larger file plus dead text. I tried it on a 380 KB letter. Small came back at 1.1 MB. I deleted that output and attached the original. “Compression made it bigger” is not a bug report. It is what happens when you photograph a page that was already efficient.

If the source is mostly text and already under the limit, stop. You cannot improve it by taking a picture of it.

Locked files and the order of operations

The compressor does not ask for a password. A locked PDF fails when PDF.js tries to open it. Unlock a copy you are allowed to open, compress that copy, then decide whether the compressed file needs a new password. Remember that locking on this site also repaints pages. Running unlock, then compress, then lock is three raster passes. Each pass softens the picture. Do it once, at the end, if the recipient only needs to view.

Page count is unchanged. Compression will not drop blank sheets. If half the megabytes are an appendix nobody on this email thread will open, split that appendix off before you compress. I cut a 28-page proposal to 16 by removing case studies that were already on the website. Balanced on the short file was 2.4 MB. Same photos, less paper, and the reader was not asked to scroll past marketing to find the fee.

Phones, fans, and the tab that never finishes

Each page allocates a canvas at the chosen scale. A large architectural sheet at scale 1.7 can be a multi-thousand-pixel bitmap. Twenty of those in a row will make a fan spin and can crash a tablet. If the status line sticks on “Compressing page 4 of 80,” the device ran out of memory, not patience. Drop to a smaller setting, or compress on a desktop, or split the sheet set and compress the subsets.

The original file is never overwritten by the download. The script saves a new PDF. You can still lose the master the ordinary way: by dragging the new file onto the old filename. I name downloads proposal-email-balanced.pdf and I do not “replace” the source. Disk is cheaper than reconstructing a vector plan from a JPEG.

A setting I would refuse

Small, on a document someone must sign, where the signature line sits over a photograph. The line breaks into blocks and the signed copy looks tampered with even when it is only over-compressed. High, or don’t compress, and send a link if the mailbox cannot take it. A muddy signature page creates arguments you did not need.

Color shifts are mild at 0.88 and obvious at 0.58 on gradients — skies, brand ramps, heat maps. If the document’s point is the color scale, treat small as a last resort and say in the email that the attachment is a reduced preview. Offer the master on request. I have had a client make a decision off a posterized legend. Once.

For slides, you may not want a PDF at all. A single page run through PDF to JPG at the image exporter’s scale 2 and JPEG 0.92 is a sharper picture than this compressor’s “small” setting, and it is still a picture. Different tool, different numbers, same family of tradeoff. Pick it when the destination is a deck, not an archive.

I keep a note in the project folder: setting, output size, whether text still needed to be selectable. Next time the same template goes out, I do not re-debate it. The master stays the master. The email file is a messenger with a short life.

A second file that behaved differently

Same afternoon I ran an invoice: 2 pages, 6.4 MB, one photo of a signed receipt and almost no text. Balanced dropped it to 1.3 MB and the signature was still recognizable at 150% zoom. Small dropped it to 640 KB and the signature turned into a gray thumbprint. I sent balanced. The invoice did not need selectable text. The proposal did, which is why the proposal’s master never got this treatment and the invoice’s master still sits next to the email copy anyway. Habit is cheaper than sorting files by “which ones were safe to destroy.”

Already-compressed scans sometimes shrink only 10%. If balanced takes 22 MB to 19 MB, you have not solved a 10 MB portal. Split or remove pages. Re-running small on an already rasterized file softens it again for a small win. I did that twice on a map and the contour labels disappeared. The first balanced export was the one I should have kept.

Name the file with the setting. invoice-balanced.pdf tells the next person why the text will not highlight. invoice-final.pdf tells them nothing and invites someone to treat paint as the record copy.