You can change font and background colour in Microsoft Word. Depending on what version of Acrobat you have, you may be able to convert your PDF to Microsoft Word. This might not be very satisfactory if your original has pictures or a complex layout.

Alternatively you can convert the non-accessible PDF into Word using SensusAccess. SensusAccess can provide additional accessible formats, such as EPUB. If the file is on moodle you can convert it using Blackboard Ally within moodle.