The available outputs depends on the server you are pointing at - so no guarantees. Rather than try to download pre-rendered maps at a given scale and then stitching them, you would be better working with one of the OSM plugins to QGIS to either
- Have OSM as a backdrop to your map by using the
Openlayers Plugin - your backdrop will zoom as appropriate with your map.
- Download the vector data and style it to suit. You can use the
Download OSM plugin to get your AOI (or go to Geofabrik etc). Then find a qml for the standard OSM styles.
To get a style file and a lot more information about using OSM in QGIS read the wiki page.
You have two more options if you insist on pre-rendered raster files for your backdrop.
- The first is to use something like leaflet-offline to download your tiles and dictate the zoom level(s). The scale will be limited to the TMS grid structure of the server you point at - and you have no control over that. See the OSM wiki for scales per zoom level (but note that the scale will change north and south with increasing distance of the equator.
- The second is to download your OSM data (perhaps using QGIS or Overpass) and the set up a Mapnik renderer.
In reality, unless you have a very specific need to go down the route you suggest, I'd go with the first two QGIS plugin options. They are the least work and will provide excellent results in most situations (the second QGIS option is the most flexible of all). The time spent looking for a server that will give you a vector PDF (rather than a TIFF/JPEG/PNG) then stiching the images is not worth it. Either of the QGIS plugin routes will have you up and running in a couple of minutes.