It’s a fact that many corporate sites still rely on an awful lot of content in PDF format, sometimes for the wrong reasons, sometimes for very good reasons. Usually it’s a fait accompli. So recently I was asked to provide file size guidelines for PDFs.
Thing is, providing a simple file size guide for PDFs is hard. There are so many variables to take into account.
Firstly, and most importantly, it depends on the file contents itself. File size is less important than comprehensibility. If detailed images in a PDF are downsampled or compressed too much, they may well lose meaning, and that would be unacceptable in, for example, a medical document. So: always check a PDF after compression.
Secondly, guidelines may differ between countries. You should try and ascertain rough broadband penetration amongst your target users in that country. If that user group has low broadband penetration, then a smaller guideline size should be aimed for. For the general UK online population, broadband penetration is over 95%. But whatever you do, avoid the spurious statistics on www.internetworldstats.com (deliberately not linked).
Finally, consider the likely use cases:
a) A user downloading a document in their own time
b) A user downloading a document in a hurry (e.g. a doctor during a consultation, or someone just checking if this is the information they want)
c) A user sending a downloaded document by email
The following time estimates should be considered bearing the above use cases in mind. Let’s take an arbitrary file size of 4 Megabyte (MB). Maximum* transfer times are:
| Dial-up (56kbps modem) |
9m18s (for downloading or emailing) |
| Typical broadband download (2Mbps) |
32s (for downloading) |
| Typical broadband upload (256kbps) |
2m36s (for sending email) |
*In practice many factors can make it up to 50% slower than this.
A positive factor to bear in mind is that PDFs in the browser will usually start displaying before they have finished downloading (provided that the PDF was created competently, but that’s another story.)
In conclusion, I recommended aiming for file sizes of around 2MB, up to a maximum of 4MB. If the file cannot be compressed below 4MB without unacceptable quality loss, consider redesigning the source material or breaking it into separate files. This does not mean that no attempt should be made to compress PDFs already under 2MB. Remember, the smaller the file size, the more users will appreciate it, no matter how fast their connections.
As an aside: I originally recommended the use of Apture, which can display dynamic previews in the page of PDFs in iPaper format, stored on Scribd. Unfortunately, due to legal issues around storing PDFs on Scribd, this was ruled out.
Recent comments