When creating a PDF for Flow there two things to consider:
- the ratio of width to length
- the PPI (Pixels Per Inch).
For the ratio you need to map it to the same ratio as an iPad to avoid borders.
A4 is too tall...
US Letter is too wide...
The resolution of an iPad is 1536 x 2048 which works perfectly...
However, what is important is the ratio of width to height which is 1.3333333...
So, whatever size you choose to create your PDF in the ratio should be 1.333333... to 1.
The second thing to consider is the PPI - if any image in your document has more pixels than can be displayed at 1536 x 2048 it means that your PDF pictures are larger than can be displayed.
This may not be an issue aesthetically as the images will just be scaled down to fit on the page - however if they are excessively larger it may mean that your PDF is larger in file size than it needs to be which will mean longer download times. The PPI of the iPad retina display works out at 72 ppi (The maths are: 2048 pixels wide on a 7.75 inch wide display > 2048 / 7.78 = 264. And 264 just happens to be the advertised resolution of the Retina iPad.)
However, two other things to consider:
- Zoom - the user can zoom into the display with a pinch motion, making things up to twice as large. A higher resolution would mean less pixelation when they do that.
- Future-proofing - who knows what resolutions Apple and other tablet manufacturers might have for future devices.
Therefore I'd suggest going for 144 dpi for any images in the PDF. Note that this only applies to bitmapped-type images (jpg, gif, png, etc.) - if you use vector graphics it doesn't apply. If you can use vectored graphics for illustrations you will get good results and save file space. You can do this using an application like Adobe Illustrator or Adobe InDesign. The output of vector graphics in these PDF's will be extremely small, as opposed to bitmap.
Test PDF files attached for reference.
KBA , BI-ROM-APP-IOS-FLOW , Roambi iOS Flow app , How To