However, in the same document you can also see that Spain is the second EU country with the most positive image of the European Union, second only to Sweden (page 10).

When asked about membership in the EU, the answer is also more positive than average, although in a more modest 9th place (http://www.europarl.europa.eu/at-your-service/files/be-heard... page 37).

So it depends on how you ask. I would say saying that Spain is the least anti EU country is quite an overstatement, but it's definitely more pro-EU than average.

I think the results of the question on trust may be biased by the fact that Spanish people don't trust politicians in general. In page 6 of this Eurobarometer survey (https://ec.europa.eu/spain/sites/spain/files/eb88_nat_es_es.... - sorry, in Spanish, I couldn't find it in English) you can see that while trust in the EU is not very high, it's quite higher than trust in the Spanish government. So most people wouldn't want to leave the EU.