First, the text of the current standard editions of Apollonius’ works—as a simple Unicode text file, as an MS Word file, and as a PDF file (which I recommend for printing). To download a file without opening it, right-click on the link and choose Save Link As… The first three treatises have been edited by Richard Schneider; the Syntax is due to Gustav Uhlig. (Publication details may be found in the Bibliography of Editions.)

 Περὶ ἀντωνυμίαϲ [.txt] [.doc] [.pdf]                
Περὶ сυνδέсμων [.txt] [.doc] [.pdf]                
Περὶ ἐπιρρημάτων [.txt] [.doc] [.pdf]                
Περὶ сυντάξεωс [.txt] [.doc] [.pdf]                

And secondly, some editions of Apollonius’ works in digital format:

Manutius A (1495) Theodori Introductiuæ gramatices libri quatuor. Eiusdem de Mensibus opusculum sanequapulchtu [sic]. Apollonii gramatici de constructione libri quatuor. Herodianus de numeris, Venice   [PDF 17 MB]
This is the first edition of Apollonius’ Syntax, and the first book of Apollonius in print. The Aldine—named after the printer Aldus Manutius—is no easy read for the novice, because it imitates the look of a Renaissance manuscript and uses a panoply of abbreviations. Before downloading the entire file, the prospective reader might thus want to try a test page—for instance, the very beginning. (For ease of reference, I have added bookmarks and numbered the pages; the Syntax starts on page 213.)
Sylburgius F (1590) Apollonii Alexandrini de syntaxi seu constructione orationis libri IIII: A Francisco Porto ante aliquot annos e manuscripto codice passim & correcti & suppleti; tum Latine redditi, & notationibus illustrati: Nunc denuo a Frid. Sylburgio cum bonis exemplaribus collati, & notationibus aucti. Addita e Procli Chrestomathia Grammatica, Photii Patriarchae CP. [i.e. Constantinopolani] Electa; ab Andrea Schotto primum edita, Latinitate donata, & scholiis illustrata; nunc iterum a Sylburgio m.s. exemplaris collatione passim emendata, suppleta, notationibus aucta. His accessere quatuor Inventaria, verborum, rerum, auctorum; tum Anomaliarum grammaticarum breuis adnotatio: opera & studio Fr. Sylburgii, Frankfurt   [PDF 26 MB]
Excellent critical edition of the Syntax by Friedrich Sylburg, with a Latin translation by François Portus. Thanks to the discovery of (an apograph of) Paris. gr. 2548, Sylburg’s text is more complete than Manutius’, for it includes Synt. 2.191.4–246.3 and 4.478.10–end. (For ease of reference, I added again bookmarks.)
Bekkerus E (1811) Apollonii Dyscoli, grammatici Alexandrini, de pronomine liber, Berlin   [PDF 17 MB]
The περὶ ἀντωνυμίαс was first edited by Emanuel Bekker in 1811. It was Bekker’s first edition; he was a mere 26 years old. In the praemonitum, Friedrich August Wolf, with Philipp Buttmann co-editor of the series, rightly praised the work of his pupil in the highest terms. Barely two years later, Bekker’s edition was reprinted. The two editions differ only minimally: text and layout are the same, but the reprint has in addition a more extensive index mendorum on the last page. The pagination is also new: in the edition of 1811, the text starts at page 261; in the reprint it starts at page 1.
Bekkerus I (1814) Anecdota Graeca, vol. 1: Lexica Segueriana, Berlin   [PDF 15 MB]
Bekkerus I (1816) Anecdota Graeca, vol. 2: Apollonii Alexandrini de coniunctionibus et de adverbiis libri, Dionysii Thracis grammatica, Choerobosci Diomedis Melampodis Porphyrii Stephani in eam scholia, Berlin   [PDF 16 MB]
Bekkerus I (1821) Anecdota Graeca, vol. 3: Theodosii canones, editoris annotatio critica, indices, Berlin   [PDF 20 MB]
Bekkerus I (1817) Apollonii Alexandrini de constructione orationis libri quatuor, Berlin   [PDF 25 MB]
After his second voyage to Paris in 1815, Bekker published what was to become the standard edition of the Syntax for the next hundred years. The work is dedicated to his friend and ξένοс Maximilien Séguier. (Bookmarks signal the beginning of each book.)
Maas P (1911) Apollonius Dyscolus. De pronominibus pars generalis, Bonn   [PDF 32 MB]
Paul Maas edited only the first part of the treatise (i.e. Pron. 3.3–49.7). His edition is based on Schneider’s: Maas had not seen the manuscript (v. praefatio). The work is notorious for the great number of deletions; the most widely discussed is at the very beginning where Maas considered it indispensable to excise a discussion of Aristarchus’ definition and of Dionysodorus’ appellation of pronouns (Pron. 3.12–3.19). Youthful and stimulating.

Help in obtaining or digitising the missing items would be most welcome.