Merged PDF too big? Find the heavy part and shrink it

Why is the merged file so big?

Merge PDF copies each page with everything it needs: its text, its fonts, its pictures. Nothing is redrawn and nothing is re-compressed, which keeps the pages exactly as they were. It also means the merged file weighs what its parts weigh.

We measured it on 4 sets of parts:

3 things stand out:

• The merge itself adds nothing. Every merged file came out a little smaller than its parts, because the result is written compactly.
• 1 scan outweighs 9 documents. In the second row the 3 scanned pages are 1.5 MB of the 1.7 MB; the 9 invoices are 195 KB together. Shrinking the invoices would change nothing.
• Shared things are not shared. When 10 parts each carry the same font or the same logo, the merged file carries it 10 times. Merge does not look for duplicates across files.

How do I find the heavy part?

Before compressing blindly, look. Inspect opens 1 PDF and shows, under Why it is this size, how much of the file is Images, the Largest image with its page and pixel size, and Everything else (text, fonts, drawings). The Fonts section lists every font and whether it is embedded.

1. Inspect the parts, or the merged file.
2. If Images is most of the file, the weight is scans or photos, and Compress will help a lot.
3. If Everything else is most of it and the Fonts list shows the same embedded font many times, the weight is fonts, and compressing will not help much (see below).

Compress after merging: what it does

Compress PDF re-encodes pictures at a lower resolution and stores identical pictures once. Text, fonts and drawings stay as they are, so typed text stays sharp and searchable.

On the merged file of 9 invoices and a scan, the result said 1.7 MB to 393 KB: 78 % smaller, and listed Merged 8 duplicate pictures: the logo, which each invoice had carried on its own, is now in the file once. The Web preset gave 762 KB, for when people will zoom in.

Steps:

1. Drop the merged file on the compressor above.
2. Pick Email (pictures at 110 dpi), Web (150 dpi) or Print (300 dpi), or type a Size target such as 10 MB for a mail limit.
3. Press Compress and read the before and after line.

For a portal or mail limit, the size target is the better tool: it finds the best picture quality that fits. Make a PDF smaller has the numbers for every lever.

Compress before or after merging?

• After in most cases: 1 run, 1 setting, and duplicate pictures across the parts are stored once, which compressing each part on its own cannot do.
• Before, when the parts need different treatment: a photo you want sharp next to scans you want small. Compress the scans as a batch, leave the photo, then merge.

The honest part: what cannot get smaller

• Fonts stay. Compress does not touch fonts, and Merge does not remove duplicate fonts. In our third row, 10 copies of the same font made 2.2 MB, and Compress only took off the duplicate logos, to 2.0 MB. If your parts were exported with full fonts, export them again with the font subset (most office programs have an option to embed only the characters used), or make the PDF in 1 export from the source documents.
• Text barely moves. A typed document is small already. Compressing a merge of typed files returns it nearly unchanged, and the tool says when a file has no pictures to shrink.
• Already compressed scans have a floor. If every part is a scan that was compressed before, merging cannot make bytes disappear, and compressing again gives little: our Email result run through Email again comes back unchanged. The floor is roughly the sum of the scans at the setting you can accept. Why scanned PDFs are huge explains where a scan’s weight sits, and compressing a scanned PDF how far it can go.
• Fewer pages is the biggest lever. If the recipient needs 3 pages of a 40-page scan, send those 3.

Merge, Inspect and Compress all run in your browser; your files are read and written on your device and nothing is uploaded.