What for processing equipment are you using? Without discribing the processing condition it is difficult to give you a good answer.
However, PLA is very prone to thermical degradation. Depending on the average molecular weight this may be very well a problem. For example a PLA grade with a MW of 100000 g/mol starts to thermal degradate around 160 - 170 °C . The thermical degradation may lead to a slight yellow coloring and can also lead to slight opaquness . In addition, the crystallinity can affect the transparency of the sample - so trying a PLA grade with a different crytallinity may help you to archieve the wanted transparency.
Also tempering your sample after the compression molding may improve transparency - i would suggest ~ 100 °C, however time and/or temperatur may vary depending on your PLA grade (molecular weight, crystallinity, D/L-ratio).
Well, void could avoid voids by good mixing. For sheet, definitely it the temperature which changes color, you can do your film preparation under nitrogen. Again, many factors are responsible for opacity including isomer types you are working.
I suspect the void formation due to air/ambient gas creeping into the system during synthesis. try making the Compression molding under vaccuum. As for opacity, the reason is, I suspect, the process of gelation and fusion, which is also called crosslinking for thermoplastics (?? surprisingly, this is apparently an industrial terminology) resulting in a crystalline matrix.
Did you dry your PLA before compression moulding ? if yes, is it dried enough ? The presence of moisture can lead to bubbles under a high temperature, and also promote a hydrolysis degradation of the PLA.
Moreover, if it is a recurrent problem, you can equip your mould with air channels which will permit air or moisture to be evacuated during the process.
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