With additional malic acid you could risk malo-lactic fermentation which will lower the acidity back down. While MLF is not a certainty, it could undo much of your having added that additional acid for taste in the first place (other spoilage issues too, potentially). You are at higher risk for this as well if the cider bulk aged on any lees bed at all.
Another couple possibilities are using citric acid or tartaric acid.
Citric acid is often used to brighten a wine. But also with a citric acid addition, technically *if* MLF occurs because of available nutrients and the malic acid from the apples, the citric also being metabolized by the bacteria could create acetic acid (vinegar) to detectable levels. Though I don't think the small citric dose you would assumedly be using is really much of a risk.
One common workaround would to use lysozyme and SO2 (k-meta etc) to prevent MLF, then you could go ahead with the acid additions and not worry about it ... but lysozyme is quite expensive and many don’t like sulfites in their cider and besides, the sulfiting will make your yeast too sluggish if you are going to bottle condition/carbonate.
Another way around this would be to use Tartaric acid as your acid addition. Technically, about 1 teaspoon per gallon should give you about 0.1% raise in Total Acidity. But especially if you are doing it by taste, I would go up in increments no larger than ½ teaspoon tartaric acid crystals per gallon at a time then re-evaluate.
If you are using 32 ounce test portions of cider to determine how much acid to add, then 1/8 or even 1/16 teaspoon increments (add-taste-add-taste-etc)
Test portions while figuring acid-to-taste is a very good idea.
In general, any time you adjust acid it is a really good idea to have and use a TA/Total Acidity test kit and also pH test tape ... I’d hesitate to raise TA above 0.5%.
If you don’t have the equipment and chemicals to measure TA, ... do it by taste ... but carefully.
For what it’s worth, in general, some variables to help prevent MLF in wine include ...
Insuring adequate levels of SO2 (particularly with low sulfite producing yeasts) ... this means keeping the pH a bit lower (i.e. more acidic) say around pH 3.4 ... and using the sulfites. (this can be an issue though if you are yeast carbonating however)
Insure there’s no extra nutrients from having added nutrients that were not used up in the initial ferment and/or nutrients that developed because the cider sat on the lees for too long.
Lower storage temps help.
Higher alcohol % helps ... though this is neither here nor there with regard to your already fermented cider.
And fine filtering to remove any residual nutrients ... but if you don’t have such a filter then this obviously would not apply.
What I would do?
First I’d be relying on testing as well as taste ...
I’d be using citric acid if the additional acidity I wanted was slight to moderate.
If the TA was really weak ... say 0.3% or below, I’d be using tartaric and citric. (I like the brightness citric imparts). And I'd be shooting for a total acidity not higher than about 0.5%
All this having been said, *if* the cider is going to be drank fairly soon (within a month or two) and *if* after carbonating you are going to keep it in the bottom shelf of the icebox ... and *if* you did not introduce large amounts of unused residual nutrients by over adding yeast nutrient (or adding it too near the end of fermentation) nor by letting the cider sit on the lees during all that aging ... the odds are you could probably get away fine with adding the malic and not get MLF *if* that’s all you had and you were willing to take the slight risk.