As you may know, Arcanists have a spellcasting method that is very similar to a spontaneous caster’s in some areas. However, they are not considered as such. That being said, there is a line in the class description stating the following: “Feats and other effects that modify the number of spells known by a spellcaster instead affect the number of spells an arcanist can prepare.”.
I wish to know whether by RAI, an arcanist can use item affecting spells known, such as a Page of Spell Knowledge, that state “being a spontaneous caster” as a requirement. It would make sense that such items, created before the introduction of the Arcanist, were written with the assumption “spells known = spontaneous caster”, and thus their ability to be used by an Arcanist would depend on the specific choice of words rather than a precise intent. For instance, in the Ring of Spell Knowledge description is written:”useful only to spontaneous arcane spellcasters”, which is not a requirement, thus it can be used by Arcanists. However the Page of Spell Knowledge have the wording:”If the bearer is a spontaneous spellcaster”, which makes it unusable. Given the similarity between the effects I find this strange.
This point seems to be further supported by the Expanded Arcana and Expanded Preparation, since the latter is a near-perfect copy of the former, worded for Arcanists. This could indicate that the authors intended Expanded Arcana to be available to Arcanists, but unfortunately it was disallowed by RAW.
Given these points, I’d be inclined to conclude Arcanists can indeed use such item by RAI, but I would like to have the judgements of more experienced Pathfinder players and DMs.