I have purchased a Synology DS1010 (5 x 2TB) for use in storing a large number of relatively small scanned documents (PDF) which will need to be retrieved at fairly frequent intervals by multiple users (not same document at same time) Documents will be stored directly from scanning, so will need a quick upload time.
Could anyone recommend a suitable RAID type? The data will be backed up to a secondary smaller NAS box 4x2TB?
For that kind of load, where you're doing mostly read but writes need to be quick, your file-system choice will probably matter more than your RAID choice. That said, the Synology DS1010 doesn't offer much choice in that respect. EXT3 seems to be your only choice, and it's OK at handling the "lots of small files" problem.
RAID5 gives you the most bang for your buck, but of course costs the most when writing. However, it sounds like writing will be a small minority of your IO operations. Read speed is what you need, and for that RAID5 is just fine for what you're looking for.
Unless this the kind of document management system where lots of docs get uploaded, but very few are ever looked at again, at which point your primary I/O type is writes. In that case, depending on the actual performance you get in the device, RAID10 is possibly more suited. Testing will tell which is what you want.
How much of this will you actually fill within the next 2-3 years? If you won't consume more than 4TB of data within the lifespan of the equipment, making it RAID10 with a hot spare seems like a no-brainer.
Otherwise, you may need to experiment with RAID5 or RAID6 (if supported) and see whether the performance is sufficient.
From a performance perspective it's irrelevant as it only has two GigE nics, from an availability point of view always go with R10, if not then R6 or R1.
Whatever your choice, to get top performance you must ensure that the partition table is aligned with the raid stripe size. I'm not sure if Synology does proper alignment by default, but evidently they use an ext3 filesystem under the hood, so it can be done right, but you may have to do so manually.