Pineapple has something of a mixed reputation as far as health goes. It is reputed to be helpful for reducing inflammation, including arthritis; it may be of help in healing mouth ulcers (canker sores), and also respiratory diseases. On the other hand, it is not recommended for those with stomach ulcers, and, far from healing canker sores, may sometimes be their cause.
An important nutrient found mainly in fresh pineapple is the enzyme bromelian. This is known to reduce swelling and inflammation, including that of conditions such as sinusitis. It is the same enzyme which ensures that gelatine will not set if mixed with fresh pineapple juice, and is entirely absent from canned or cooked pineapple.
If you suffer from any mild cold or sinus infection, it is probably worth eating some fresh pineapple for a few days, if it is readily and inexpensively available. If nothing else, it will give you a little extra Vitamin C and other important nutrients that may be lacking during a viral or bacterial infection. It may also soothe inflamed bronchial tubes.
Arthritis suffers may like to eat pineapple daily over a month or two, perhaps topped with some powdered ginger, to see if any improvement results. Research is not conclusive, but it is unlikely to do any harm, unless you also have an internal ulcer.
As for mouth ulcers, clearly you should avoid pineapple - particularly under-ripe pineapple which can be highly acidic - if it seems to cause them. However, well ripened juicy fresh pineapple pieces have been known to soothe and even heal small canker sores.
A cup (8 fluid ounces) of pineapple contains roughly 180mg magnesium, 20g each of magnesium and calcium, 13mg phosphorus, and trace of other minerals. It also contains nearly 80mg Vitamin C, almost 100iu Vitamin A, and traces of some B vitamins, particularly folic acid.