You're not the first to have this problem: a lot of tall 'tower' infographics can be very awkward to re-use. There's a bit of a trend away from them since they can be awkward on social media outside of blogs, tumblr etc as well (they can also be associated, often unfairly, with the trashy end of the market where graphics get long due to lack of quality control rather than an excess of interesting information).
If you've got one of the high quality ones that is long for good reason, here's a few possible options:
- PDF attachments: Regular PDFs can have attachments - other files embedded in the PDF. There's also such a thing as 'PDF Portfolios' but my experience of these is that they're a bit of an inflexible powerpoint-inspired early 2000s monstrosity. I have no idea if ISSUU supports PDF attachments (you should maybe ask for their advice directly - post an answer here if they suggest any useful tips). I did a short write-up on PDF attachment options for a slightly similar question about including raw data in PDFs, which might help.
- Ask about reorganising it: Well-designed multi-message infographics usually organise their messages and information into bite-size chunks that lead into each other in some kind of narrative. With the original designer and/or copyright owner's permission (and you should be in contact with them anyway to get permission to re-publishing their work), you may be able to divide the infographic's chunks into panels which can be organised to run across multiple pages of your magazine. Be careful though that you don't ruin it... Some designers might volunteer to quickly do the chopping up and re-organising for you to make sure it looks right to their standards (others never would, or would charge to, varies from person to person).
- Pull out and magnify sections of interest: If the purpose of including the graphic is to illustrate or demonstrate certain points, have it scaled down on the page as a central column, and use the whitespace on either side of the tower for magnified call-outs for particularly interesting chunks. Then include an easy-to-type short-form link (e.g. bit.ly/some-graphic) to the full thing. So, people get an idea and get the point you're making, and can then go see the full thing if they're interested.
Do what books do: Look at how infographics showcase books get around this problem, and talk to ISSUU about the best way to do the digital equivalent. Since these are things the print industry does, someone is bound to have asked before and they should have some suggestions:
- Rotate it: Placing the graphic landscape across a double-page spread (bare in mind though that it's easier for a reader to turn a book around than a computer monitor)
- Fold out spreads: Having a fold-out spread for particularly large ones, again rotating the image if necessary. PDFs support varied page spreads, but talk to ISSUU about whether their digital platform does.
Some books get around this problem by being absolutely absurdly massive... (Image from the blog ixycreativity.ro). Not sure if that's an option or not...